Method for forming a temperature sensing layer within a thermal barrier coating

ABSTRACT

A thermal barrier coated component, such as a turbine blade formed from a superalloy substrate, includes a thermal barrier coating applied onto the substrate. A metallic bond coat layer is on the substrate and includes rare-earth luminescent dopants. A ceramic top coat layer is on the bond coat layer. A temperature sensing thermally grown oxide (TGO) layer is formed at the interface of the bond coat layer and ceramic top coat layer. The temperature sensing TGO layer includes grown rare-earth luminescent ions migrated from the metallic bond coat layer in an amount sufficient to enable luminescence sensing of the TGO layer for real-time phosphor thermometry temperature measurements at the TGO layer.

PRIORITY APPLICATION(S)

This is a divisional application based upon U.S. patent application Ser.No. 17/034,065 filed Sep. 28, 2020, which is based upon U.S. provisionalapplication Ser. No. 62/944,390 filed Dec. 6, 2019; and based upon U.S.provisional application Ser. No. 62/940,963 filed Nov. 27, 2019; thedisclosures which are hereby incorporated by reference in theirentirety.

GOVERNMENT SPONSORSHIP

This invention was made with Government support under Agency ContractAward Number DE-FE0031282 awarded by the U.S. Department of Energy,National Energy Technology Laboratory, University Turbine SystemsResearch (UTSR). The Government has certain rights in this invention.

FIELD OF THE INVENTION

This invention relates to Thermal Barrier Coatings, and moreparticularly, to Thermal Barrier Coatings that enable luminescencesensing and high temperature measurements of a substrate, such as aturbine component.

BACKGROUND OF THE INVENTION

Thermal barrier coatings (TBCs) are used to protect substrates, such asturbine components from extreme environments and allow the associatedturbine systems to operate often at temperatures beyond the meltingpoint of the underlying substrate, for example, a superalloy turbineblade. These coatings may be formed as multilayers, and include in anexample, the metal substrate, a metallic bond coat, a Thermally GrownOxide (TGO) and a ceramic topcoat, such as formed from Yttria-StabilizedZirconia (YSZ). Some of these coatings are used in combination withactive cooling systems, which allow for temperature drops through theceramic top coat, in the order of 1° C./μm. Accurate measurement ofcoating temperatures in these environments ensures good performance andfunctionality of the turbine system and helps predict the lifetimeexpectancy of the turbine blades and related turbine components.

The range of uncertainty in temperature measurements should be reducedto a few degrees at service temperatures because failure mechanisms arethermally driven. This is particularly important due to the extremesensitivity at the interface between the top coat and a bond coat andany intervening layers, such as the thermally grown oxide. Currently,more viable techniques for non-contact, in-situ temperature measurementson thermal barrier coatings are: a) infrared thermometry, whereprecision is limited by the presence and variations in the emissionsfrom the operation of the turbine engines, and b) phosphor thermometry.Other existing in-situ temperature measurement techniques for hightemperature evaluation have inherent uncertainties and possibly poorsafety margins. Improving the accuracy of temperature measurements onthe materials in operating conditions is important for more reliablelifetime expectancy predictions of high temperature substrates such asturbine blades.

Direct temperature measurement at the thermally grown oxide, forexample, remains impractical and prediction models are used to helpestimate the health of the coatings. In addition, the production ofmultilayer thermal barrier coatings is both expensive and detrimental tothe mechanical integrity of the thermal barrier coatings. Currenttemperature measurement systems do not permit direct temperaturemeasurements on the thermally grown oxide layer. Instead, they enableindirect temperature approximations using a sensor coating placed in thetop coat, which may add manufacturing costs, impart potentialmodifications to the thermal properties of the top coat, and createmultilayer interfaces that can negatively affect the integrity of thethermal barrier coatings.

Phosphor thermometry has potential for improved temperature measurementcapabilities, and has been effective to enable remote temperaturesensing. This measurement technique uses phosphors, which luminescenceis due to rare-earth or transition metal ions that have been illuminatedby an ultraviolet lamp or laser source. Improved thermal barriercoatings that extend the range of temperature sensing for extremeoperating conditions are desirable.

SUMMARY OF THE INVENTION

This summary is provided to introduce a selection of concepts that arefurther described below in the Detailed Description. This summary is notintended to identify key or essential features of the claimed subjectmatter, nor is it intended to be used as an aid in limiting the scope ofthe claimed subject matter.

A thermal barrier coated component includes a substrate and a thermalbarrier coating applied onto the substrate. A metallic bond coat layeris on the substrate. The metallic bond coat layer may include rare-earthluminescent dopants. A ceramic top coat layer is on the bond coat layer.A temperature sensing thermally grown oxide (TGO) layer may be formed atthe interface of the bond coat layer and ceramic top coat layer. Thistemperature sensing TGO layer may include grown rare-earth luminescentions migrated from the metallic bond coat layer in an amount sufficientto enable luminescence sensing of the TGO layer for real-time phosphorthermometry temperature measurements at the TGO layer.

In an example, the rare-earth luminescent dopant is selected from thegroup consisting of samarium, erbium, europium and dysprosium. Theceramic top coat layer may comprise a ytrria-stabilized zirconia (YSZ)barrier top coat layer on the bond coat layer. The substrate maycomprise a turbine component or an engine exhaust component. Themetallic bond coat layer may include 96 to 98 percent of NiCoCrAlY and 2to 4 percent of dysprosium.

In yet another example, the TGO layer nay comprise about 1.7 to 2.0weight percent of nickel, about 0.67 to 0.82 weight percent of chromium,about 52.7 to 64.4 weight percent of aluminum, about 35.0 to 42.7 weightpercent of oxygen, and no more than about 0.1 weight percent of arare-earth element selected from the group consisting of samarium,erbium, europium and dysprosium. The bond coat layer may be about 50 to200 micrometers and the ceramic top coat layer may be about 50 to 300micrometers.

In yet another example, a thermal barrier coated component may include asuperalloy substrate and a thermal barrier coating applied onto thesuperalloy substrate. The thermal barrier coating may comprise ametallic bond coat layer on the superalloy substrate. The metallic bondcoat layer may include rare-earth luminescent dopants. Anytrria-stabilized zirconia (YSZ) barrier top coat layer is on the bondcoat layer, and a temperature sensing thermally grown oxide (TGO) layermay be formed at the interface of the bond coat layer and YSZ barriertop coat layer. A temperature sensing TGO layer may include grownrare-earth luminescent ions migrated from the metallic bond coat layerin an amount sufficient to enable luminescence sensing of the TGO layerfor real-time phosphor thermometry temperature measurements at the TGOlayer.

A method of forming a thermal barrier coated component may compriseapplying a metallic bond coat layer onto a substrate, said metallic bondcoat layer including rare-earth luminescent dopants, applying a ceramictop coat layer on the bond coat layer, and forming a temperature sensingthermally grown oxide (TGO) layer at the interface of the bond coatlayer and ceramic top coat layer by heat ageing the metallic bond coatlayer and ceramic top coat layer to migrate rare-earth luminescent ionsfrom the metallic bond coat layer into the interface of the bond coatlayer and the ceramic top coat layer in an amount sufficient to enableluminescence sensing of the TGO layer for real-time phosphor thermometrytemperature measurements at the TGO layer.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages of the present invention willbecome apparent from the Detailed Description of the invention whichfollows, when considered in light of the accompanying drawings in which:

FIG. 1A is a schematic sectional diagram of a thermal barrier coatingshowing the gradient of temperature and the emerging luminescence as aconvoluted signal coming from all locations in the doped layer.

FIG. 1B is a graph showing the collected signal for the emergingluminescence shown in the diagram of FIG. 1A.

FIG. 1C shows a modified Kubelka-Munk model to predict decayed behaviorof luminescence in accordance with a non-limiting example.

FIG. 2 is a graph illustrating the depth at which phosphor thermometrymay make temperature measurements in a thermal barrier coating.

FIG. 3 is a sectional block view of a thermal barrier coating showingthe doped top coat on a bond coat and substrate and showingdiagrammatically temperature measurement levels in accordance with anon-limiting example.

FIG. 4 is a graph showing the temperature sensitive region in theoperating temperature range of a gas turbine and lifetime decay.

FIG. 5 is a schematic flow diagram showing stages of oxidation for anexample thermal barrier coating and the thermally grown oxide anddysprosium impurities.

FIG. 6 is a scanning electron microscopy (SEM) image of an oxidizedthermal barrier coating at 1100° C. for 200 hours.

FIG. 7 is an enlarged view of a portion of the SEM image of FIG. 6 .

FIG. 8 is a schematic diagram of a first step for forming a sensinglayer at the YSZ/bond coat interface and showing deposition of the dopedbond coat in accordance with a non-limiting example.

FIG. 9 is a schematic diagram showing the standard top coat depositionafter depositing the bond coat in FIG. 8 .

FIG. 10 is a schematic diagram of temperature sensing the thermallygrown oxide via phosphor thermometry probing in accordance with anon-limiting example.

FIG. 11 is an example of the modified Kubelka-Munk model extension fortwo layers having distinct optical properties with the absorption andscattering of luminescence light emerging from the rare-earth dopedthermally grown oxide.

FIG. 12 is a graphical model of the modified Kubelka-Munk modelextension showing the light reflection and transmission at theinterfaces.

FIG. 13 shows integrated Fresnal equations for the modified Kubelka-Munkmodel extension of FIGS. 11 and 12 .

FIG. 14 is a graph showing the measurable luminescence relative to thepercent of incident light intensity and depth of the thermally grownoxide relative to the top coat in accordance with a non-limitingexample.

FIG. 15 is a schematic diagram of an air plasma spray system that may beused to form the thermal barrier coating in accordance with anon-limiting example.

FIG. 16 is a table showing an example of compositions of the thermalbarrier coating using the air plasma spray system of FIG. 15 .

FIG. 17 is a table showing examples of the fabricated air plasma sprayeddoped thermal barrier coating configurations.

FIG. 18 is another table showing other examples of air plasma sprayeddoped thermal barrier coating configurations.

FIG. 19 is a schematic diagram showing an example of electron beamphysical vapor deposition (EB-PVD) that may be used in accordance with anon-limiting example.

FIG. 20 is an SEM image showing the EB-PVD zirconia layer and bond coaton the substrate such as a turbine blade.

FIG. 21 is a table showing examples of fabricated EB-PVD samples.

FIG. 22 is a block diagram of an example phosphor thermometry system forthermally grown oxide luminescence sensing in accordance with anon-limiting example.

FIG. 23 is a schematic diagram showing deposited layers for a thermalbarrier coating configuration that includes a luminescence layer ofYSZ:Er,Eu.

FIG. 24 is a graph showing the emission spectrum of YSZ:Er,Eu under 532nm laser excitation and the range of wavelengths collected for theluminescence measurements.

FIG. 25 is a schematic diagram of an example phosphor thermometry devicethat allows synchronized monitoring of luminescence in accordance with anon-limiting example.

FIG. 25A is another example of a phosphor thermometry device thatincludes four photomultiplier tubes.

FIG. 26 as a graph showing the room temperature decay of the R1-line ofalumina.

FIG. 27 is a schematic diagram of an induction heating system and along-wave infrared camera for sensing thermal barrier coatings using thedevices of FIGS. 25 and 25A in accordance with a non-limiting example.

FIG. 28 is a graph showing the synchronized collection of erbium andeuropium decays at 500° C.

FIG. 29A is a graph showing an example of the luminescence decays oferbium with respect to temperature at 545 nm.

FIG. 29B is a graph showing an example of the luminescence decay ofeuropium with respect to temperature at 590 nm.

FIG. 30 is a graph showing the lifetime decay response of air plasmasprayed co-doped YSZ:Er,Eu and showing the high temperature sensitivityregion.

FIG. 31 is a composite graph showing the luminescence intensityvariation with respect to temperature for erbium and europium dopants.

FIG. 31A is a sample showing convoluted luminescence of europium inaccordance with a non-limiting example.

FIG. 31B is a graph showing the europium luminescence spectrum andhighlighting two main peaks that can be separated and collectedsimultaneously such as using the devices of FIGS. 25 and 25A inaccordance with a non-limiting example.

FIG. 32A is a sectional schematic view of the thermal barrier coatingwith the doped layer at the top surface and showing a delamination.

FIG. 32B is a second example of the thermal barrier coating similar tothat of FIG. 32A but showing the doped layer at the bottom of the topcoat and showing a delamination.

FIG. 33 is a block diagram of an example delamination monitoring systemusing luminescence contrast.

FIG. 34A is a diagram showing a first modified Kubelka-Munk modelingapproach and boundary conditions to model laser and luminescenceintensities in thermal barrier coatings.

FIG. 34B is a diagram showing a second modified Kubelka-Munk modelingapproach to solve for laser and luminescence intensities and accountingfor a high scattering zone close to the bond coat in thermal barriercoatings.

FIG. 34C shows a matrix used to help determine optical properties in aspecific layer Z.

FIG. 34D shows a matrix used to help define the amount of luminescencegenerated in a specific layer Z.

FIG. 34E shows matrices and equations used to help solve for theintensity of laser and luminescence lights, as a function of coatingdepth, respectively.

FIG. 34F are equations used to help define the diffuse externalreflectivity at the interface between the top coat and the air gap orbond coat and the integrated average of the Fresnel equation to obtaindiffuse internal reflectivity.

FIG. 34G is an equation to help solve the frustrated angle-averagereflectivity for radiation with angle of incidence greater than thecritical incident angle.

FIG. 34H are equations to help determine values in perpendicularpolarization and parallel polarization.

FIG. 34I is an equation to help solve the frustrated angle-averagereflectivity for unpolarized radiation.

FIG. 34J is an equation to help solve for the error gap width dependentdiffuse internal reflectivity for an examination of early stages ofdelamination.

FIG. 35A is a photograph of a thermal barrier coating with Rockwellindentation-induced delamination or spallation areas and correspondingluminescence intensity maps of the europium line at 562 nm with thedoped layer at the top surface.

FIG. 35B is another photograph similar to that shown in FIG. 35A, butwith the doped layer at the bottom of the top coat.

FIG. 36 is a graph showing the distribution of laser intensity in thetwo-layer model of FIG. 34A showing where there is delamination, higherintensities are scattered due to increased reflection and showing thelocation of the doped layers in the examples of FIGS. 32A and 32B.

FIG. 37 is a graph showing the distribution of luminescence intensity inthe two-layer model where the doped layer is on the top coat.

FIG. 38 is a graph showing the distribution of luminescence intensity inthe two-layer model where the doped layer is at the bottom of the topcoat.

FIG. 39 is a graph showing the distribution of laser intensity in thethree-layer model of FIG. 34B and where there is delamination, higherintensities are scattered back due to increased reflection.

FIG. 40 is a graph showing the distribution of luminescence intensity inthe three-layer model where the doped layer is on top.

FIG. 41 is a graph showing the distribution of luminescence intensity inthe three-layer model where the doped layer is at the bottom of the topcoat.

FIG. 42 is a graph showing modeled signal trade-off for an embeddedsensing layer in an EB-PVD thermal barrier coating.

FIG. 43 is a schematic diagram of an example apparatus used to determinetemperature gradients across a thermal barrier coating and showing theconfiguration of the thermal barrier coating.

FIG. 44 is a graph showing the emission spectrum of YSZ:Er andapplicable for determining example temperature determinants.

FIG. 45 is a composite graph and equation showing the gradient oftemperature relative to depth in a thermal barrier coating and thesummation of the thermal gradients.

FIG. 46 is a graph showing the normalized intensity versus time when thefitting window size of the acquired signal is varied.

FIG. 47 is a more detailed schematic diagram of the example apparatussuch as shown in FIG. 43 for measuring the temperature gradients acrossa sample of the thermal barrier coating.

FIG. 48 is a graph showing a determination of reference decay in anisothermal case.

FIG. 49 is a graph showing the decay measurements in a thermal gradientcase.

FIG. 50 is a graph showing the results for phosphor thermometry,sub-surface measurements with lifetime decay in microseconds on thevertical axis versus surface temperature.

FIG. 51 is a graph showing the through-thickness thermal profile andmeasured temperature, surface temperature, and window size.

FIG. 52 are composite graphs and a schematic diagram of a thermalbarrier configuration showing how the acquisition window changes toretrace the temperature gradient into the into the thermal barriercoating.

DETAILED DESCRIPTION

Different embodiments will now be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsare shown. Many different forms can be set forth and describedembodiments should not be construed as limited to the embodiments setforth herein. Rather, these embodiments are provided so that thisdisclosure will be thorough and complete, and will fully convey thescope to those skilled in the art.

In accordance with a non-limiting example, an improved rare-earth dopedthermal barrier coating bond coat configuration has been developed andis effective to enable luminescence sensing, for example, by ultravioletlight illumination at the TGO (thermally grown oxide). The rare-earthdoped thermal barrier coating bond coat configuration allows for bettercontrol of thermal parameters in components, such as turbine engines, toallow the engines to operate more efficiently with increased safety.This improved configuration allows for direct measurement at thethermally grown oxide layer that forms in the thermal barrier coating.This process for luminescence sensing and the developed rare-earth dopedthermal barrier coating bond coat configuration allows directmeasurement at the top coat and the bond coat interface, e.g., at theTGO layer, which is the location of interest for lifetime monitoring ofthermal barrier coatings. This sensing layer may form through hightemperature oxidation. In a non-limiting example, there is norequirement to modify the industrial deposition procedure when formingthe thermal barrier coating on a substrate, such as a turbine blade. Theprocess and configuration also conserves the integrity of the thermalbarrier coating. No additional mechanical interfaces are required and nomodification of the industrial top coat of the thermal barrier coatingis necessary.

The improved process and rare-earth doped thermal barrier coating bondcoat configuration, in accordance with a non-limiting example, may beused for thermal barrier coating real-time temperature measurements,thermal barrier coating lifetime predictions, and thermal barriercoating delamination detection. In an example, a modified Kubelka-Munkmodel as explained in greater detail below is applied to determineluminescence decay behavior in the doped TBC configurations. The processand rare-earth doped thermal barrier coating bond coat configuration maybe used with phosphor thermometry measurement techniques on an operatinggas turbine engine via an optical port. The process and rare-earth dopedthermal barrier coating bond coat configuration may work even where theluminescence signal is weak and where there are uncertainties on therare-earth diffusion rates and the uncertainty in the bond coats andbond coat adhesion properties. It is also possible to quantify thermalbarrier coating delamination through luminescence modeling using themodified Kubelka-Munk model.

In an example, a component, such as a substrate for a turbine component,may include a turbine substrate, such as a turbine blade, and a thermalbarrier coating on the turbine substrate. The thermal barrier coatingmay include a bond coat layer, such as a metallic bond coat, on thesubstrate. In an example, the bond coat layer includes rare-earthluminescent ions as a dopant, for example, and a Ytrria-StabilizedZirconia (YSZ) barrier top coat over the bond coat. A Thermally GrownOxide (TGO) layer is formed at the interface of the bond coat materialand YSZ barrier top coat. The addition of the rare-earth luminescentions, i.e., a rare-earth luminescent dopant, enables luminescencesensing of the TGO layer. In an example, the rare-earth luminescentdopant is selected from the group consisting of samarium, erbium,dysprosium and europium. Other dopants may be used and it may bepossible to use chromium, magnesium and similar materials. It ispossible to use various transition metals.

Thermal barrier coatings are often applied to a substrate, such asturbine components, to protect these turbine components, such as turbineblades operating at high temperatures. Thermal barrier coatings oftenare used in combination with active cooling systems that allow fortemperature drops via the ceramic top coat. State-of-the-art thermalbarrier coatings are not being used to their greatest potential becauseof the uncertainties with measuring high temperatures, where the safetymargins may be as high as 200° C. The ideal Brayton cycle efficiency is1−T^(c)/T^(t), which corresponds to the temperature ratio across thecompressor from the exit to the turbine inlet. A 1% efficiencyimprovement may save $20 million in fuel over a combined-cycle plantlife, and a 130° C. increase may lead to a 4% increase in engineefficiency. Failure mechanisms may be driven by temperature conditionsin the depth of the thermal barrier coating. By having more accuratetemperature monitoring of the thermal barrier coatings, it is possibleto increase the efficiency of the turbine and combined engine cycle. Therare-earth doped thermal barrier coating bond coat configurationovercomes these failure mechanisms that are thermally activated duringengine operation and the uncertainty in temperature measurements thatmay reduce lifetime operation and uncertainty.

Other techniques have been tried, but are not as successful.Thermocouples are inexpensive and have a wide temperature range and havebeen proposed, but have drawbacks of intrusive probe designs, disruptedflow patterns, chemical instability, low accuracy, and incompatibilityon rotating surfaces. Infrared thermometry has a wide temperature rangeusing non-contact techniques at a fast response time, but requiresoptical access and is emission sensitive.

Phosphor thermometry, on the other hand, is a non-contact technique thathas high sensitivity at high temperatures, a fast response time, usableon rotating parts, and low sensitivity to the turbine environment, suchas aging and contamination. For example, the time dependent intensitymay be measured following an excitation pulse to determine atemperature-dependent decay time, where dopants are rare-earth elements.Embedding the doped layer in a thermal barrier coating enables atemperature measurement.

In accordance with a non-limiting example, a luminance decay behavior inrare-earth doped thermal barrier coating bond coat configurations ismeasured by using a modified Kubelka-Munk model. The classicalKubelka-Munk model provides a only luminescence intensity where themodeling decay is important to understand the effect of the thermalbarrier coating configurations.

Referring now to the schematic of the rare-earth doped thermal barriercoating configuration in FIG. 1A, the thermal barrier coating is showngenerally at 100, and includes the top surface 104, top coat 106, andbond coat 108 over which the top coat is placed in this example. A laser110 and detector 112 are also illustrated. The gradient of thetemperature exists in real operating conditions and the emergingluminescence via the laser 110 is a convoluted signal coming from allthe locations in the doped layer and showing the doped layer 120 in thetop coat 106. The laser 110 emits the signal, and the emergingluminescence is a convoluted signal to the detector 112 as shown by thespread signal. The graph shown in FIG. 1B illustrates the time on thehorizontal scale and shows the intensity on the vertical scale with thetime relative to the collected signal and measured parameters and themeasured time relative to different time periods. The modifiedKubelka-Munk model as explained in greater detail below predicts a decaybehavior of luminescence. Reference is made to FIG. 1C illustrating themathematical sequence.

Referring now to the graph of FIG. 2 , it is evident that the gradientof temperature may be calculated using thermal conductivities ofmaterials where the function may be determined from the temperaturedistribution across the coating. The differences for YSZ:Dy, i.e., addeddysprosium, or added europium (YSZ:Er) or added samarium (YSZ:Sm) areshown with the different lines.

FIG. 3 in turn illustrates a rare-earth doped thermal barrier coatingbond coat configuration 124 on a substrate 126, such as a turbine blade,and showing the doped top coat 106, bond coat 108 and substrate. Themodified Kubelka-Munk model predicts the equivalent position, indicatingat which depth the phosphor thermometry system may make its measurementsas shown by the dashed line indicated as X measured. The doped top coat106 is shown in this example as about 250 micrometers (μm) and themeasured temperature (T measured) is shown relative to the measureddistance, such as 50 micrometers. Thus, a temperature is measured ordetermined relative to depth.

The decay constant for the collectible luminescence has a value betweenthe decay constants of luminescence from the edges. The decay constantof the collectible luminescence may then be determined by a fit. Thedecay constant also infers the temperature of a particular position thatdominates the signal. This position may be determined from retracing thetemperature dependence of the decay constant, which may be fitted forany configuration that is associated with a particular position of thetop coating 106. In an example, the decay constant had been found tomatch with that of the luminescence generated at 37 micrometers in anexample, and in the case of a thermal barrier coating 100 with a dopedlayer of thickness 50 micrometers and positioned at 50 micrometers, thedecay constant may be the same as the luminescence from the positiondepth of 69 micrometers.

The modified Kubelka-Munk model allows for collecting intensitycomparisons between different configurations having the same dopant. Themodified model may predict the expected decay behavior from any phosphordoped configuration and predict the equivalent position of thetemperature measurement. This modified model may be used for collectingaccurate temperature measurements in real operating conditions, such asfor a coated turbine blade in a gas turbine, at high-temperature usingphosphor thermometry. The results of this modified model may be used forevaluating different geometrical configurations of thermal barriercoatings with phosphor thermometry and screen different phosphors to aidin selecting dopants. The results of the modified model may help reducethe costs of experiments that are otherwise required for evaluatingmultiple doped layer configurations and use of different materials, suchas different dopants.

In accordance with a non-limiting example, a thermally grown oxide (TGO)in the thermal barrier coating is part of an innovative bond coatconfiguration where the thermally grown oxide has temperature sensingcapabilities. Temperature may be measured at this location where failuremechanisms occur such as in gas turbines. The temperature may bemeasured non-intrusively into a depth of the thermal barrier coating. Inan example, the logarithmic growth may be limited by low oxygendiffusivity through the thermally grown oxide. The graph shown in FIG. 4illustrates a temperature sensitive region in the operating, temperaturerange of a gas turbine engine as shown between the dashed lines and asdefined between the circled portions. Lifetime decay is shown on thevertical axis and temperature on the horizontal axis and showing aluminaas Al₂O₃ and chromium and dysprosium dopants as the material. Inoperational use, the temperature drives the oxide growth in the thermalbarrier coating on a turbine blade, for example, and is a major factorin coating failure.

In accordance with a non-limiting example, rare-earth luminescent ions,e.g., rare-earth dopants, may be added to the bond coat 108 (FIG. 5 ) ofthe thermal barrier coating to enable luminescent sensing at thethermally grown oxide. Reference is made to the article: Liu, Xiaoju, etal., “Microstructural Evolution and Growth Kintetics of Thermally GrownOxides in Plasma Sprayed Thermal Barrier Coatings,” Progress in NaturalScience, Materials International, 26.1 (2016) 103-111, where anexplanation is given for oxidation stages in duplex thermal barriercoatings, but without rare-earth dopants. On contrast to Liu et al., theaddition of these dopants allows an accurate measurement of coatingtemperatures in extreme environments to ensure and maintain goodperformance, functionality of the turbine system, and allow predictionestimates of the lifetime left of the turbine blades. A rare-earth dopedthermal barrier coating bond coat for thermally grown oxide sensing isformed. An example of the thermally grown oxide, which may include Dy(Dysprosium) impurities is shown in the schematic diagram of FIG. 5 ,illustrating different stages of oxidation in a multi-layer thermalbarrier coating and includes an undoped top layer 128, such as formedfrom YSZ and on the bond coat 108, which is on the substrate 126 such asInconel 718, corresponding to an austenitic nickel-chromium alloy thatis often used, for example, in turbine components such as turbine bladesand exhaust components that take high heat. The oxide growth occurs fromelements of the metallic bond coat 108 and oxygen from the surface. TheCoNiCrAlY superalloy is used in the example at the bond coating 108 suchas applied by air plasma spray. The Inconel 718 alloy 126 has been knownto form a thick, stable, passivating oxide layer when heated. Adding Dy(Dysprosium) is usually not accomplished using its oxide form (Dy₂O₃)because the oxide form may not migrate properly into the thermally grownoxide illustrated at 130 to form the thermally grown oxide that includesin this example the rare-earth dopant Dy. The oxide form may bedetrimental to the bond coat. The middle drawing sequence in FIG. 5shows the Al³⁺ initially at the interface for migration, and thesequence on the right shows the Dy³⁺, Al³⁺, CO²⁺, Ni²⁺ and Cr³⁺ at theinterface for migration. Other rare-earth ions could be used.

Referring now to FIGS. 6 and 7 , there are shown scanning electronmicroscopy (SEM) images 132 of a fractured cross-section of the oxidizedthermal barrier coatings at 1100° C. for 200 hours at a low rateaccording to Liu et al. This example has the Al³⁺ ions and other ionsfrom the bond coat 108 migrating to the thermally grown oxide 130. TheCr³⁺ may be responsible for the emission of R-lines in the Al₂O₃.Energy-dispersive x-ray spectroscopy (EDS) may be used for analysis. AnEDS (energy-dispersive x-ray spectroscopy) compositional analysis ofweight percent (wt. %) of three points in the fractured cross-section ofthe oxidized thermal barrier coating and thermally grown oxide 130 after200 hours at 1100° C. includes no cobalt, but according to Liu et al.,at point 1 shown in the enlarged section of FIG. 7 , includes nickel atabout 1.87 weight percent, chromium at about 0.74 weight percent,aluminum at about 58.58 weight percent, and oxygen at about 38.81 weightpercent.

However, the added dysprosium in accordance with a non-limiting exampleadds traces in this non-limiting analysis to the area of point 1. Thisconfiguration as described with the added rare-earth element in thisexample such as the dysprosium is advantageous because measurements mayoccur at the YSZ ceramic top coat and bond coat interface. No additionalmechanical interface is required and there is no requirement to modifyan industrial YSZ thermal barrier.

In an example, the metallic bond coat layer includes about 96-98% ofNiCoCrAlY and about 2-4% of a rare-earth element or ion, such asdysprosium. In another example, the TGO layer includes about 1.7 to 2.0wt. % of nickel, about 0.67 to 0.82 weight percent of chromium, about52.7 to 64.4 wt. % of aluminum, about 35.0 to 42.7 wt. % of oxygen, andno more than about 0.1 weight percent of a rare-earth element selectedfrom the group consisting of samarium, erbium, europium, and dysprosium.These elements may be in ion form and other materials may be used,including praseodymium and terbium. Other transition elements may beused. These base values above may vary as much as up to 5%, 10%, 15%, or20%, and other incremental variations and all values in between thesevalues. The bond coat may be about 50 to 200 micrometers. The ceramictop coat layer may be about 50 to 300 micrometers. The exampledysprosium may be no more than about 0.1 wt. % but can vary by amountsas little as 1% to as much as 20% of that 0.1 wt. % and values inbetween for the TGO.

Referring now to FIGS. 8-10 , there are illustrated a sequence of stepsfor forming a sensing coating or layer as a temperature sensingthermally grown oxide that allows temperature measurement at theYSZ/bond coat interface. In FIG. 8 , the doped bond coat 150 isdeposited on the substrate 154, such as a turbine blade, and may includeas a non-limiting example NiCoCrAlYDy, and forms the rare-earth dopedbond coat. In FIG. 9 , the industrial YSZ coating 156 is deposited asthe top coat. The doped bond coat deposition process 151 could be plasmaair spray or other deposition techniques. At the next step as anon-limiting example, the thermally grown oxide 160 containing theAL₂O₃:CR,Dy is shown in FIG. 10 to form the sensing layer as atemperature sensing thermally grown oxide, and illustrates the regularin-service condition with possible sintering and measuring temperatureusing the phosphor thermometry probe 164 as part of a thermometrysensing system.

The modified Kubelka-Munk model of luminescence that emerges out of thisrare-earth doped thermally grown oxide 160 as shown in FIG. 10 , isillustrated in the modified Kubelka-Munk model shown in the modelexpansion and matrix of FIG. 11 , and in the thermal barrier coatinglayer schematic of FIG. 12 . The Kubelka-Munk model extension isoperative with a sample having two layers of distinct opticalproperties, but multiple layers may be employed. The model expansion inFIG. 11 shows a first expanded matrix for the absorption and scatteringof luminescence light while traveling and coupled with a second matrixfor the production of luminescence. The resulting model expansionculminates in the graph at the right side showing the percent ofincident intensity for I_(laser) and J_(laser) on the vertical axis anddepth in micrometers relative to the top coat 156 and thermally grownoxide (TGO) 160.

The physical parameters in the schematic cross-section model of FIG. 12shows the top coat 156, thermally grown oxide 160, and a bond coat 150with the laser signal and luminescence. The thermal barrier coatingdepth is illustrated on the horizontal axis. The reflection andtransmission at the interfaces shown in the model of FIG. 12 wereestimated using integrated averages of Fresnel's relations (FIG. 13 ).The measurable luminescence 167 is illustrated in the graph of FIG. 14with percentage of incident intensity shown on the vertical axis anddepth in micrometers for the thermally grown oxide 160 relative to thetop coat 156 on the horizontal axis and showing the rise in the percentincident intensity for I_(lum) and J_(lum) in the top coat 156 andchange at the thermally grown oxide 160 showing the increase in theI_(lum) and decrease in the J_(lum).

Different deposition methods may be used to form the thermal barriercoatings as described, including an air plasma spray (APS) device (FIG.15 ) and electron beam physical vapor deposition (EB-PVD) device (FIG.19 ) as explained in greater detail below.

Referring now to FIG. 15 , there is illustrated a schematic diagram of apowder injector system as an air plasma spray device at 200, forexample, for air plasma spray (APS) and having an injector 202 thatinjects powder at a particle size D₅₀ in this example of about 1-100micrometers and a torch 204 that applies Argon (Ar) and helium andhydrogen (He/H₂) combination to vaporize and accelerate particles ontothe substrate 154 such as a turbine blade, and showing the APS splats208. The plasma temperature may be about 10⁴ K and the system mayoperate with a plasma flow velocity of about 10³ meters per second(m/s), and the values may range as much as ±10%.

One aspect of this described air plasma spray device 200 is the possibleformation of interlamellar pores and unmelted particles on the substrate154, which may form voids and include oxide inclusion. Any highoxidation rate may damage luminescent properties, and thus, this processmay be controlled to establish a desired thickness and minimize theimparted roughness of deposited layers. There is also no ideal powderparticle/shape for deposition.

Referring now to the table 210 in FIG. 16 , there is shown an examplecomposition to be synthesized as NiCoCrAlYDy and different compositionalparameters at the YSZ layer for 8% Y₂O₃ and 92% Zr₂O₃ and thecomposition to be synthesized in this example as 97% NiCoCrAlY and 3%Dy. These values may vary from 5%, 10%, 15% or 20% as upper and lowervalues and all values in between these values. The requirements on thedoped bond coat, may be about 30 micrometers (D₅₀) particle size for thespray to be fed properly and a minimum quantity of about 25 grams. Thetables of FIGS. 17 and 18 show examples of the fabricated APS doped bondcoat configurations, as non-limiting examples, with the table values inFIG. 17 corresponding most closely with the table values of FIG. 16 .Different specimens are shown and example coupon dimensions. Referencesamples use the same spray parameters.

Referring now to FIG. 19 , there is shown an example Electron BeamPhysical Vapor Deposition (EB-PVD) device as a system at 220 and showingschematically the EB-PVD vacuum chamber 224 and an electron beam 226injected into the YSZ ingot 230 from electron beam source 227 andforming an evaporation plume 232, which allows the columnar growth 236on the preheated substrate 154, such as a turbine blade. The chemicalbonding helps form the smooth bond coat surface with reduced oxidationunder vacuum conditions, and imparts better control over the depositionrate and the resulting chemistry. The EB-PVD vacuum chamber 224 may beoperable at about 10⁻⁶ O₂ atmospheres, and the substrate 154 preheated(T_(s)) to about 1000° C. The deposition rate may vary, but in oneexample, is about 10 μm/min (micrometers per minute).

An example SEM image of a typical thermal barrier coating 238 producedby an example EB-PVD system is shown in FIG. 20 and shows an EB-PVDzirconia layer 240 and bond coat 242 on the substrate 244. Reference ismade to Galetz, Mathias C., “Coatings for Superalloys,” Superalloys,InTech, 2015, 277-298. This article explains generally the examplelayers.

Referring to FIG. 21 , there is shown a table at 246 for EB-PVD samplesand the compositional analysis of the YSZ and YAG:Er and GAP:Cr. Thetable 246 illustrates the different specimens and the sensing layer asthe EB-PVD YAG:Er or EB-PVD GAP:Cr and the sensing layer located on topof the top coat, below the top coat or within, or the whole bond coat.

The rare-earth doped bond coat configuration as described is effectiveto enable luminescence sensing at the thermally grown oxide. Thisconfiguration also allows for better control of thermal parameters, forexample, in turbine engines, to operate them more efficiently withincreased safety. This configuration allows for direct measurement atthe thermally grown oxide layer that forms naturally in the thermalbarrier coating. The top coat and the bond coat interface at the TGOlayer, which is the location of interest for lifetime monitoring ofthermal barrier coatings, may be monitored because the sensing layer mayform naturally through high temperature oxidation. There is norequirement to modify the industrial deposition procedure and theintegrity of the thermal barrier coating is conserved. No additionalmechanical interfaces are required and no modification of the industrialtop coat thermal barrier coating is required.

In the example of a gas turbine, the thermal barrier coating is formedon the turbine substrate and includes the bond coat, which includes arare-earth luminescent dopant, and a Ytrria-Stabilized Zirconia (YSZ)barrier top coat over the bond coat. The Thermally Grown Oxide (TGO)layer at the interface of the bond coat and YSZ barrier top coat enablesluminescence sensing of that TGO layer. In an example, the rare-earthluminescent dopant may be selected from the group consisting ofsamarium, erbium, dysprosium, and europium, but other rare-earthelements may be used and other dopant ions and materials, includingtransition metals.

Phosphor Thermometry Device for Synchronized Acquisition of LuminescenceLifetime Decay and Intensity on Thermal Barrier Coatings

It is possible to include two or more dopants or ions and simultaneouslycollect multiple emission peaks, such as upon excitation by a laserpulse. A phosphor thermometry temperature measuring system as a deviceis shown generally at 250, in accordance with a non-limiting example(FIG. 22 ) and collects two emission peaks simultaneously, in thisexample of an Erbium and Europium co-doped Yttria-Stabilized ZirconiaThermal Barrier Coating (TBC), enabling extended temperature range andhigh precision of in-situ temperature assessments. The device 250captures the luminescence lifetime decays and the intensity variationsof both dopants, thus allowing testing with high sensitivity andextended temperature range capabilities for accurate measurements, up tothe operating temperatures of turbine engines. In an example, the system250 is portable and performs effective temperature monitoring on turbineengine materials and advanced innovative sensing coatings. The system250 for testing generally includes a support 254 such as a cart forportability, a photomultiplier device such as a PMT (photomultipliertube) power supply 258, laser power supply 260, oscilloscope 264, laser268, IR camera 272, and first and second photomultiplier devices, inthis example, photomultiplier tubes 274 a, 274 b (PMT1 and PMT2). Acontroller 280 is operative with these components and controls theiroperation and functions.

This system 250 extends the capabilities of phosphor thermometry bypartitioning the luminescence signal and specific reflection of selectedelectromagnetic spectrum bands for the synchronous acquisition ofluminescence decay from different electronic transitions. The system 250may perform real-time high-temperature measurements on luminescentthermal barrier coatings (TBC) and extend the range of measuredtemperatures used in extreme environments, such that the system 250 maybe configured for in situ operation and adapted for real life operationof gas turbine engines, for example.

The system 250 is capable of selectively partitioning theelectromagnetic spectrum from an emitted luminescence signal andreflecting the separated luminescence peaks into the respective firstand second photomultiplier devices as the photomultiplier tubes 274 a,274 b in this example for individual luminescence decay acquisition. Thesystem 250 collects distinct decay characteristics from multipleelectronic transitions occurring in the same point of the probedmaterial and provides data for more precise temperature measurements inextreme environments. In an example, the system 250 may use dichroicfilters as explained later relative to the schematic diagram of FIG. 25, to split and reflect the luminescence signal into the first and secondphotomultiplier tubes 274 a, 274 b, which convert the received photonsto a signal that is traced on the oscilloscope 264. The resulting decaysmay be computed via a program at the controller 280 that converts thedecays to temperatures.

This system 250 enables higher and more precise temperature measurementsand extends the temperature range over which those measurements mayoccur. Experimental results indicate the range of temperature may beextended from 25-900° C. to a greater range of 25-1100° C. with enhancedprecision, using, for example, a YSZ:Er,Eu phosphor. The additional costresulting from the addition of at least one photomultiplier tube, suchas a second photomultiplier tube (PMT2) 274 b in this example, andassociated optics is small when compared with the cost of the entiresystem 250. This system 250 may be used as a substitute for currentphosphor thermometry instruments or other devices, and it may replacealternative measurement techniques that use, for example, infrared orthermocouple technology, which may carry higher error levels in extremeenvironments.

In operation, during the measurement process, a single laser pulseemitted from the laser 268, for example about a 10 ns pulse, may sufficeto obtain multiple decays that can be correlated to the temperature atone point. The controller 280 may be configured to process the datagathered at the oscilloscope 264 and retrace the temperature fromluminescence.

The phosphor thermometry system 250 in this non-limiting examplecollects synchronized data of the luminescence decay and processes thatdata to obtain an intensity ratio as measured from the two independentemission peaks, such as emanating from an Erbium-Europium co-doped,Yttria-Stabilized Zirconia air plasma spray thermal barrier coating.Although experiments were accomplished on thermal barrier coatingsapplied by air plasma spray, other application techniques may be used,such as the EB-PVD system 220 described above. The system 250 maycollect the luminescence emerging out of the doped layer up to 1100° C.with 50° C. incremental steps as an example, while the surfacetemperature may be concurrently measured. Higher resistances at theinput of the oscilloscope 264 may amplify the signal-to-noise ratio andallow the system 250 to collect lifetime decays with a sufficientbandwidth up to 850° C. At higher temperatures, a limited response timemay be compensated by the acquisition of a ratio of intensity betweenErbium and Europium emission peaks, which may vary due to the fasterquenching of Europium. As a result, the range of temperatures that maybe accurately measured using rare-earth doped YSZ configurations areextended up to gas turbine engine operating temperatures.

The simultaneous acquisition of phosphor thermometry data may permitmore precise measurements with extended temperature ranges using ahigh-sensitivity decay process, combined with a high detectabilityintensity ratio. The synchronized acquisition capabilities of thisphosphor thermometry system 250 provides efficient in-situ temperaturemeasurement options for turbine components that operate at hightemperatures.

As noted before, the thermal barrier coatings protect the gas turbinecomponents operating at high temperatures. These thermal barriercoatings are generally used in combination with active cooling systemsthat allow for temperature drops through the ceramic top coat, in theorder of 1° C. μm⁻¹. Accurate measurement of the coating temperatures insuch extreme environments ensures and maintains good gas turbineperformance, ensures functionality of the system, and helps to predictthe lifetime of the turbine blades. The temperature measurementuncertainty is preferably reduced to a few degrees at servicetemperatures because failure mechanisms are thermally driven, which maybe important due to the extreme sensitivity of the growth rate of thethermally grown oxide to the temperature at the interface between a topcoat and the bond coat. Thus, a sensor layer or coating for hightemperature measurements is formed and integrated into thermal barriercoatings using the use of rare-earth doped yttria-stabilized zirconia tooffer sensing capabilities.

Europium-doped YSZ (YSZ:Eu³⁺) has excellent temperature sensitivity pastits quenching temperature of about 500° C., and a visible luminescenceand a longer room temperature decay time. Similarly, erbium-doped YSZ(YSZ:Er³⁺) has a strong visible luminescence intensity, a shorter roomtemperature decay time, and a temperature sensitivity between roomtemperature and the elevated turbine operating temperatures. With ausable absorption band at about 532 nm and distinct emissionwavelengths, both dopants are used together in this example in aco-doped configuration that combines their properties for accurateassessment of turbine blades and other components.

The system 250 as developed and explained with reference to FIG. 22permits the luminescence produced by YSZ:Er³⁺ and YSZ:Eu³⁺ to beisolated and simultaneously collected. The data is doubled forprecision, combining sensitivities of the dopants and extending thetemperature range at which the system 250 may measure in situ thetemperature on rare-earth doped YSZ thermal barrier coatings. Both thedecay and intensity ratio processing techniques may be used to takeadvantage of the synchronized collection of two dopants. The testedsample in this example contained the sensing layer with its codopedeuropium and erbium at its top surface as shown in the configuration inFIG. 23 , and the phosphor thermometry measurement was compared withinfrared thermometry, and the configuration allowed for a strongerluminescence intensity from the sample.

Description of the Fabrication Process. Samples were fabricated using anSGT-100 (Praxair) spray gun at the air plasma spray (APS) facility ofthe Florida Institute of Technology. The materials and parameters usedfor the deposition of the layered configuration are given in Table 1shown below. The thermal barrier coating configuration is shown at 300in FIG. 23 and includes the metallic bond coat layer 304, and a ceramictop coat layer 306 on the bond coat. The ceramic top coat layer 306includes an undoped layer 308, and doped sensing layer as the upper ortop coat 312 for the sensing layer, and all positioned on and over thesubstrate 316, for example, a one-inch of CM247 alloy. Exampledimensions are in micrometers, and in this sample, the doped upper layeras the doped sensing layer 312 was about 80 micrometers, the undopedlayer 308 was about 250 micrometers, and the bond coat 150 was about 150micrometers.

These dimensions may vary. For example, the doped sensing layer 312 maybe about 70 to 90 micrometers and the undoped layer 308 may be about 230to about 270 micrometers. In an example, the doped sensing layer 312forms the top layer of the TBC over the undoped layer 308 and may beformed as an erbium-europium co-doped Yttria-Stabilized Zirconia (YSZ).The erbium concentration in the YSZ may be about 1.25 to 1.75 wt. %, andin an example, 1.5 wt. %. The europium concentration in the YSZ may beabout 2.5 to 3.5 wt. %, and in an example, about 3.0 wt. %. The firstphotomultiplier device 274 a may be configured to detect erbium spectrallines at about 545 nm and 562 nm. The second photomultiplier device maybe configured to detect europium spectral lines at about 590 nm and 606nm. All these values can vary by as much as 5%, 10%, 15% or 20% aboveand below the stated values and ranges and values in between thosevalues.

TABLE 1 Materials and Parameters for the Air Plasma Spray DepositionUndoped Doped Layer Bond Coat Top Coat Top Coat Material (mixing NiCrAlYYSZ 66% YSZ + 17% percentages, given YSZ—Er (1.5% in wt. %) Er) + 17%YSZ:Eu (3% Eu) Thickness (μm) 150 250 80 Spray Distance (cm) 10 7.5 7.5Current (A) 802 902 902 Voltage (V) 43.3 43.7 43.7 Argon (SLM) 49.1 25.525.5 Helium (SLM) 20.3 20.8 20.8 Feeding Wheel Speed 1.17 3.29 0.48(rpm)

In an example, the substrate as a coupon and used for experimentationwas formed as a 25.4 mm diameter and 3 mm thickness CM247 disk. Thesample was grit blasted prior to depositing the bond coat layer 304. Astud was welded on the back of the substrate 316 to mount the sample ona deposition stage. NiCrAlY bond coat powder (NI-164/NI-211, Praxair),7-8 wt. % YSZ undoped top coat powder (ZRO-271, Praxair) and erbium- andeuropium-doped YSZ top coat powders, as produced by a solid statereaction from Phosphor Technology Ltd., were used for the deposition ofthe respective layers 304, as shown in the air plasma sprayed thermalbarrier coating configuration 300 of FIG. 23 , which includes theluminescent YSZ:Er,Eu layer 312. In this configuration 300 of FIG. 23 ,the dimensions are in micrometers (μm), and include the APS depositedYSZ:Er,Eu layer 312; APS deposited YSZ layer 308; APS deposited NiCrAlYbond coat 304; and the CM247 substrate layer 316, corresponding to apolycrystalline nickel superalloy often used for gas turbine blades.

An erbium concentration of 1.5 wt. % in the YSZ was chosen for optimalluminescence intensity. The europium concentration of 3 wt. % was chosenfor high luminescence intensity and that amount limited dopantintrusiveness to prevent phase change. The amounts can vary by about upto 5%, 10%, 15% or 20%, and all values in between, and can range from0.5 to 2.5 wt. % erbium to 2.0 to 4.0 wt. % europium in yet anotherexample. The pre-processing doped YSZ powders initially had a smallerparticle size (D₅₀<1 μm) and irregular particle shapes, and for thatreason, the powders were mixed together with the undoped YSZ powder toensure the flowability of the mixture and a good deposition rate for theuppermost layer. The particle size can vary. A feeding wheel as part ofthe deposition process had its speed decreased to obtain a constantdeposition rate with reduced clogging. It was determined that a mixingratio of about 1:2 for the doped powder and the undoped powder wasoptimal to obtain a good deposition rate, while maintaining sufficientdoped material for luminescence intensity. The fabricated thermalbarrier coating 300 was examined using scanning electron microscopy onits microstructure with secondary electrons and an accelerating voltageof 15 kV. The uninterrupted and successive deposition of undoped anddoped YSZ layers ensured the uniformity of the overall top coat with novisible interface. The sample was annealed for 2 hours at 800° C. toremove possible luminescence quenching compounds.

Spectral Characterization of the Sample. The emission spectrum of theYSZ:Er,Eu sample was measured with a collection time of 1 ms using afiber collection spectrometer (Pixis 100, Princeton Instruments) under a15 mW 532 nm laser excitation. The probe had a focal length of 7.5 mm, adepth of field of 2.2 mm, a numerical aperture of 0.27, and a spot sizeof 200 μm. The Er-lines at 545 nm and 562 nm and the Eu-lines at 590 nmand 606 nm were observed. The co-doping may have introduced some levelof reabsorption of the erbium lines due to the presence of europium thatpossessed an absorption band that excited the ⁵D₁ level and extendedfrom 520 to 550 nm. This could ultimately result in a smaller intensityratio between erbium and europium. In this experiment, the peak oferbium at 545 nm (⁴S_(3/2)→⁴I_(15/2)) and the peak of europium at 590 nm(⁵D₀→⁷F₁) was chosen for luminescence intensity and decay measurements.

The graph in FIG. 24 shows the emission spectrum of the YSZ:Er,Eu sampleunder 532 nm laser excitation, highlighting the two regions that werecollected by the phosphor thermometry detectors and showing the 543.5 nmbandpass between the lines labeled “A,” and the 590 nm bandpass betweenthe lines labeled “B.” The full width at half maximum of the bandpasseswas represented to indicate the range of wavelengths collected for theluminescence measurements.

General Specifications of the Phosphor Thermometry Device. The phosphorthermometry device as shown in the block diagram of FIG. 22 is adaptableto many temperature sensor materials that have at least two emissionpeaks that can be isolated by dichroic filters. The device may beportable and mounted on a stable cart 254 (FIG. 22 ). The height may beadjustable by either lifting the top surface of the cart or individuallylifting the example first and second photomultiplier tubes (PMTs) 274 a,274 b, which are mounted on precision lab lift jacks 275 and fixed on amain optical breadboard 276 that is connected to the controller 280 in anon-limiting example.

A schematic block diagram of portions of the phosphor thermometry system250 described in FIG. 22 is shown schematically in FIG. 25 and permitssynchronized monitoring of luminescence from YSZ:Er,Eu. Similarreference numerals as used with reference to FIG. 22 are used in thisdescription of the block diagram of FIG. 25 . The system 250 is equippedwith the YAG:Nd pulsed laser (CNI) 268 that provides 10 ns of laseroutput at a frequency of 10 Hz. The radiation emitted by the laser 268may be a fundamental mode at 1064 nm, and in an example, includes apulse energy of about 2 mJ. The light travels through afrequency-doubling potassium titanyl phosphate (KTP) crystal, forexample, to generate 532 nm pulses on the order of 0.5-1 mJ. The 1064 nmand 532 nm beams are collinearly traveling through a lithium triborate(LBO) crystal to generate pulses at the sum frequency of 355 nm, withpulse energies up to 0.5 mJ. The first and second photomultiplier tubes274 a, 274 b may be formed as two Hamamatsu R3896 tubes powered by thePMT power supply 258 (FIG. 22 ) as a single precision direct currentpower supply providing 15 volts and a maximum current of 90 mA for eachdetector. The maximum internal resistance of a gain control module ofeach PMT 274 a, 274 b was measured to be around 80 kΩ.

For this experiment, the 532 nm excitation was chosen and the 355 nm wasstopped with a beam blocker. First and second 532 nm laser mirrors 282a, 282 b adjusted the direction of the beam to the sample correspondingto the thermal barrier coating configuration 300 described relative tothat sample shown in FIG. 23 . After the laser beam was correctlyaligned in the axis of the sample, a cyan dichroic filter 284(FD1C-Thorlabs) was placed in the path of the beam so the angle ofincidence (AOI) was 30°. This value was chosen because it lowers thereflection of the laser light on the cyan dichroic filter 284 and allowsfor a higher excitation intensity onto the sample 300. A magentadichroic filter 286 (FD1M-Thorlabs) was then placed at an angle ofincidence of 45° from the axis of reflection of the cyan dichroic filter284. This allowed the convoluted luminescence of the co-doped sample 300that was reflected on the cyan dichroic filter 284 to be split furtherinto two spectral bands.

The shorter wavelengths containing the erbium emissions were reflectedon the magenta dichroic filter 286 and directed to the firstphotomultiplier device as PMT1 (photomultiplier tube number 1) 274 alocated in the axis of reflection of the magenta dichroic filter. Thelonger wavelengths containing the europium emissions were transmittedthrough the magenta dichroic filter 286 to the second photomultiplierdevice as PMT2 (photomultiplier tube number 2) 274 b, located in theaxis of reflection of the cyan dichroic filter 284. The distance fromthe cyan dichroic filter 284 to the sample 300 was about 30 cm and thedistance between the two dichroic filters 284, 286 was about 8 cm. Forthe collection of the decays, the two PMTs 274 a, 274 b were usedsimultaneously. On PMT1 274 a, connected to channel 1 of theoscilloscope 264 (FIG. 22 ), a 543.5 nm (10 nm FWHM-Thorlabs) bandpassfilter was placed in the viewing port of that detector. Because thewavelengths of the erbium emission at 545 nm and the excitation at 532nm were close to each other, some laser leakage occurred due to laserreflections passing though the bandpass filter, which had a transmissionand optical density of about 532 nm at 0.19% and 2.73, respectively.

Because the laser intensity was very high in comparison with theluminescence intensity, to better protect the first photomultiplier tube274 a and avoid the undesired collection of laser light, a longpassfilter having an optical density of five (5.0) on the 190-532 nm rangewas added. On the second photomultiplier tube (PMT2) 274 b, connected tochannel 2 of the oscilloscope 264, a 590 nm (10 nm FWHM-Thorlabs)bandpass filter was mounted in the viewing port. To avoid directexposure to intense laser reflections, prevented by the collection ofexternal light, and to capture light traveling through the opticalcomponents of the system 250, a laser barrier panel was placed in frontof the system. A viewing hole was extruded to insert an iris, opened toits maximum aperture (25 mm) and a 125 mm convex lens shown at 290,which converges the slightly divergent laser beam onto the sample 300.This arrangement resulted in a spot size of about 4 mm on the surface ofthe sample, placed at the focal distance of the convex lens 290, andallowed for the collimation of the luminescence light traveling to thedetectors.

Initial Test. The system 300 was tested using a known R-line (ruby)emitted from an alumina block under a 532 nm excitation pulse. Thisexperiment was performed at room temperature using a 694.3 nm bandpassfilter (10 nm FWHM-Thorlabs) for the specific collection of the R1-linedecay. The cyan dichroic filter 284 was used to transmit the laser beamto the sample 300 and to reflect the luminescence signal to thedetector, which in this example is illustrated generally at 275,correspond to the PMT's. The resulting decay was fitted using a singleexponential model and is shown in the graph of FIG. 26 , illustratingthe room temperature decay of the R1-line of alumina and showing thenormalized intensity on the vertical axis and the time in seconds on thehorizontal axis and the fit where T was equal to about 3.3889 ms.

High Temperature Setup. In this example, high temperature was achievedusing an induction heater coil 292 (RDO HU2000), which produced the highfrequency, pulsating magnetic fields to induce internal and circulatingeddy currents in the sample material 300 causing resistive heatingwithin the material. A frequency of about 272 kHz and a lift-offdistance of 5 mm was used between the induction coil 292 and the surfaceof the sample 300, as shown in the schematic drawing of FIG. 27 ,illustrating a high temperature set-up using the induction principle andtemperature control and a long-wave infrared camera 272 (FIG. 27 ). Anenlarged view of the pulsed excitation laser onto the sample 300 showsthe induction heating coil 292 and schematically the circulating eddycurrents 293 and the magnetic field 294 and their location relative tothe sample 300 having the thermal barrier coating. The resistive heatingQ generated by the internal eddy currents is described by equation:

$Q = {\frac{1}{\sigma} \cdot {J_{s}}^{2}}$

where σ is equal to electrical conductivity and J_(s) is the eddycurrent density generated by the magnetic field. In order to maintainthe sample surface parallel to the induction heating coil 292 and normalto the horizontal path of the laser beam, a circular segment of the diskwas cut-off so the sample can rest on refractory blocks. Temperatureincrements of 50° C. were achieved up to 1100° C. by increasing thepower of the induction heating system. Phosphor thermometry data wascollected at each temperature step. Induction heating was chosen forthis experiment because it did not produce background thermal radiationthat facilitated luminescence measurements.

Control of the Temperature Using Infrared Thermometry. The temperaturewas measured using a TIM450 (Micro-Epsilon) longwave infrared camera(7-13 μm) 272 operating at about 30 Hz and placed at 20 cm from thesample 300, which corresponded to the focal distance of the camera. Thecollection area was reduced to match the phosphor thermometry laser spotsize. The camera 272 used a program referred to as “TIM Connect”thermography software to track the thermal radiation emitted from thesample. The emissivity was set to €=0.95 for all readings as this valuecorresponded to the emissivity of YSZ in the long-wave infrared rangeand did not vary noticeably with temperature.

The description has proceeded with reference to the system 250illustrated in FIGS. 22 and 25 that includes first and secondphotomultiplier tubes 274 a, 274 b. However, third and fourthphotomultiplier tubes 274 c, 274 d may be incorporated into the system250 as shown in the example system of FIG. 25A and include the bandpassfilters at each photomultiplier tube 274 a-d and additional dichroicfilters 285.

Acquisition of Luminescence Decays. The luminescence decays of theerbium emission at 545 nm and the europium emission at 590 nm werecaptured simultaneously, using both first and second photomultipliertubes (PMT1 and PMT2) 274 a, 274 b respectively. The Siglent SDS 1204X-Eoscilloscope 264 converted the electric signal to data matrices. Infront of the viewing port of PMT1 274 a, the incoming light included aspectral band, which ranged in wavelength between 532 nm and 560 nm. Itcontained the luminescence emitted by erbium and the laser reflections.This light followed the optical path originating at the sample 300surface, which was reflected on the cyan dichroic filter 284 withR_(cyan,545 nm)≈95%, and reflected on the magenta dichroic filter 286with R_(magenta,545 nm)≈92%. The spectral band was narrowed to selectthe peak of erbium at 545 nm, using both a 543.5 nm bandpass filter anda laser cut-off longpass filter. In front of PMT2 274 b, the incomingsignal contained a spectral band with a wavelength range between about560 nm and 720 nm. It contained the europium luminescence that emergedout of the sample surface and was reflected on the cyan dichroic filter284 with R_(cyan,590 nm)≈100%, and transmitted through the magentadichroic filter 286 with T_(magenta,590 nm)≈91%.

The specific luminescence emission peak of europium at 590 nm wasselected by placing the 590 nm bandpass filter described before in theviewing port of PMT2 274 b. To obtain a greater amplitude of the signalthat facilitated the detection of the luminescence decay and increasedthe signal-to-noise ratio (SNR), fixed resistance loads were connectedto RG58 coaxial cables that linked the PMT detectors 274 a, 274 b to theoscilloscope 264. Resistances of 50 kΩ and 5 kΩ were applied to theacquisition channels associated with PMT1 274 a for erbium and PMT2 274b for europium, respectively.

The graph shown in FIG. 28 shows an example of the simultaneous decayacquisition, captured at 500° C. and showing the long decay of europium(≈632 μs) compared to erbium (≈13 μs), and the higher noise of theeuropium decay due to the lower resistance value chosen for theacquisition. In this experiment, the amplitude of the signal was favoredover the time response to enable the use of luminescence intensities upto the higher temperatures. By selecting these higher feedthroughresistances, the minimum lifetime decay that may be accurately measuredwas found to be about 6 μs for channel 1 and about 1.5 μs for channel 2.

After the completion of the experiment, the collected data was processedusing MATLAB code that synchronized all decays so that t=0 correspondedto the start of the decay at all temperatures, as presented in thegraphs of FIGS. 29A and 29B, showing the luminescence decays of anerbium emission at 545 nm (FIG. 29A) and europium emission at 590 nm(FIG. 29B). The MATLAB code used a single exponential model to fit thedata of both decays. Even though the europium decay was shown to have atriple-exponential decay behavior, the single exponential model had beenwidely used because it offered a more robust fit for the temperaturemeasurement using this phosphor.

Measurement of Luminescence Intensities. The use of first and secondPMTs 274 a, 274 b and the high signal-to-noise ratio allowed for thecomparison of the intensities at t=0, when the laser excitation pulseended and where the luminescence intensity reaching the detectors wasmaximal. The first and second PMTs 274 a, 274 b received a reducedregion of the spectrum that was passed by the bandpass filters andconverted the photons to an electrical amplitude, measured in volts. Thequantum efficiency of first and second PMTs 274 a, 274 b was expected tobe slightly higher at 545 nm (≈20%) than at 590 nm (≈18%). For thisexperiment, the feedthrough resistance of channel 1, which collected theluminescence of erbium, was set to a higher value than the resistance ofchannel 2, as it allowed amplitudes exceeding one volt for both dopants.At room temperature, the integrated intensity of the luminescence ofeuropium between 585 nm and 595 nm was found to be about 55% higher thanthe integrated intensity of erbium between 538.5 nm and 548.5 nm in thespectral ranges of the bandpasses. This was primarily due to theparticular quantum yield of erbium and europium in YSZ, with thespecific concentration and distribution parameters that were obtainedfor this sample.

The collected intensities were processed by the MATLAB code to subtractthe growing thermal radiation background from the measurements, andaverage the intensity received by the PMTs 274 a, 274 b when laserexcitation is off and no luminescence remains. This takes placetypically before a new excitation pulse, which corresponds on theoscilloscope 264 window to the trace preceding the rise-time and decay.The ratio between the normalized intensities of the two dopants may beused for high temperature measurements, assuming that the spectral shiftdue to temperature is negligible in comparison with the spectralbandpass of the filters used for the collection of light. The integratedintensity measured by each of the two PMTs 274 a, 274 b is, inconsequence, assumed to remain proportional to the peak intensity ofeach radiative transition.

Temporal Analysis. The independent luminescence decays were fitted ateach 50° C. step and the graphs of the results are shown in FIG. 30 ,showing the lifetime decay response of the air plasma sprayed co-dopedYSZ:Er,Eu. The error bars represented the standard deviation of threeindependent measurements that were performed with an identical setup, onthree different locations onto the sample. It is evident that the decayof erbium at 545 nm is sensitive to temperature over the entire range oftemperature from room temperature to 850° C. with a lifetime decay timeat room temperature of about 31 μs. The decay of europium had a veryhigh sensitivity in the range of temperatures in between the quenchingtemperature of the phosphor in YSZ (≈500° C.) and 850° C., with alifetime decay time of approximately 1.2 ms at room temperature. Thehigh temperature sensitivity region of the europium is shown by theenclosed region at 400.

The advantages of the co-doped configuration and simultaneous dataacquisition of the decays was the extension of the temperature range onwhich the decay technique was applicable as the individual dopants havedifferent maximum sensitivity temperature ranges. Past 850° C., theeuropium decay indicated a lifetime that stagnated around 1.5 μs, whichcorresponded to the detection limit of the system 250 on this channel asrelated to the resistance used for collection of the signal.

This inaccurate lifetime decay was generated by the limited response ofthe system 250 at high temperature. Reducing the resistance feedthroughon the channel may have reduced the response time, but it implied asignificant degradation of the signal-to-noise ratio. A compromisebetween the signal-to-noise ratio and time-resolution may be selectedfor resulting high temperature phosphor thermometry. The sensitivity ofthe decay of europium appears to be high between 500° C. and 850° C.,which corresponded to the luminescence quenching of the dopant into YSZ,due to the higher probability of a vibrational de-excitation past thequenching temperature. However, the sensitivity of the decay of europiumoutside this highly sensitive temperature range was close to zero. Thecombination of europium with erbium, facilitated by the temperaturemeasuring system 250, allows for the extension of the range oftemperature that can be measured using the decay technique due to thelifetime decay of Erbium, which may be differentiated for anytemperature between room temperature and the turbine system operatingtemperatures in a non-limiting example.

Intensity Considerations. The luminescence intensity variation wasmeasured and the ratio between the intensities of the erbium emission at545 nm and the europium emission at 590 nm was measured, which werenormalized with respect to their distinct room temperature intensity.This data was obtained to provide additional information for thetemperature measurement using the phosphor thermometry system 250. Theluminescence intensity was obtained by measuring the maximum of theamplitude of the luminescence decay, which was proportional to theamount of photons reaching each PMT 274 a, 274 b at the moment where thelaser excitation pulse produced the strongest luminescence. As thetemperature increased, it was determined that the luminescence intensityof both dopants initially increased, due to the thermal filling of theenergy levels, and then decreased due to thermal quenching.

The growing thermal radiation, which spectral radiance can be predictedusing Planck's law, provided a contribution to the overall intensity ofwhich one part is the luminescence. The percent variation of theluminescence intensity for each dopant, with respect to the roomtemperature intensity, is shown in the composite graphs of FIG. 31 ,showing the percent luminescence intensity variation with respect toroom temperature intensity for each dopant and the corresponding ratioand sensitive between 850° C. and 1100° C., and thus, usable for hightemperature measurements. The error bars indicate the standard deviationof three independent measurements that were performed with an identicalset-up, on three different locations onto the sample. It was also foundthat the ratio of the normalized intensity variation of erbium toeuropium, R_(Er/Eu), had a sensitive range of temperature, usable from850° C. up to at least 1100° C., as presented in the lower plot andshowing the extended temperature range as defined by the enclosedportion at 410. The error bars indicate the propagation of error fromthe ratio of the intensity variation between erbium and europium. Thisis a result of the fast quenching of europium luminescence on this rangecompared to the luminescence of erbium, which remained strong, as shownin the three-dimensional plots to the right of the graphs. Although thelifetime decay of europium could not be accurately determined past 850°C. due to the limitation on the temporal resolution of the device, thehigh signal-to-noise ratio allowed for valid intensity measurements athigher temperatures.

Measurement Error Considerations. To evaluate the precision in thesystem 250 and obtain a representative response of the system with theYSZ:Er,Eu sample 300, three independent measurements were performed. Thephosphor thermometry system 250 set-up remained unchanged while thesample and the induction coil 292 were translated normally to the laserto collect the luminescence signal from three different locations. Theintrinsic temperature measurement uncertainty was implied by the use ofinfrared thermometry, whose accuracy is ±2% at high temperature. Thematerial was brought to the desired temperature values for datacollection using infrared thermometry as the reference to determine thepower input of induction heating.

Another limiting factor when targeting a temperature for measurement wasdue to the gradient of temperature in the doped layer, created by theheat flux from the top surface to the bond coat 304 in the thermalbarrier coating, but which direction was inverted due to use of theinduction coil 292 that preferentially heated the metallic substrate anddissipated heat by convection at the free surfaces of the material.Furthermore, material characteristics varied with the position probed onthe sample. For example, the random distribution of porosity affectedboth thermal and optical properties of the material and the irregularityof the doped layer thickness contributed to possible variations in theluminescence of the sample. A possible uneven distribution of dopantinto the material may possibly intensify ion-ion interaction or theconsecutive thermal cycling that results in intensity and lifetime decayvariation when probing different locations onto the doped layer.

Uncertainty on the exponential fit may also be significant, inparticular with europium, which typically exhibits a triple-exponentialdecay behavior. The precision of the measurements at 800° C. was foundto be ±8° C. and ±3° C. using erbium and europium decays, respectively.In addition, a lab-scale prototype necessitated automated acquisitionfor calibration to reduce experimental errors.

The phosphor thermometry system 250 is operable for the synchronizeddata collection of the luminescence decay and intensity ratio measuredfrom two independent emission peaks, coming from an erbium-europium,co-doped yttria-stabilized zirconia air plasma spray (APS) thermalbarrier coating. The luminescence emerging out of the doped layer 312(FIG. 23 ) was collected up to 1100° C. with 50° C. steps, while thesurface temperature was concurrently measured using the longwaveinfrared camera 272. The high resistances at the input of theoscilloscope 264, chosen to amplify the signal-to-noise ratio, allowedthe collection of lifetime decays with a sufficient bandwidth up to 850°C.

At higher temperatures, the limited response time was compensated by theacquisition of the ratio of intensity between erbium and europiumemission peaks, which varied significantly due to the faster quenchingof europium. As a consequence, the range of temperature that could beaccurately measured using the rare-earth doped YSZ configurations wasextended up to turbine engine operating temperatures. The simultaneousacquisition of phosphor thermometry data allowed more precisemeasurements on extended temperature ranges using the high sensitivitydecay process combined with the high detectability intensity ratioprocess. Exploiting the synchronized acquisition capabilities of thisnovel phosphor thermometry system 250 provided efficient in-situtemperature measurement options for turbine components.

As shown in FIGS. 31A and 31B, it is possible to use a common phosphorfor synchronous decay acquisition, such as an erbium doped thermalbarrier coating. As shown in FIG. 31A, the coupon 400 has an erbiumconvoluted luminescence with a convolution spacing as illustrated ofabout 2.5 mm. The graph in FIG. 31B illustrates the erbium luminescencespectrum and highlights the two main peaks that can be separated andcollected simultaneously to obtain the higher precision results thanusing a single peak.

Synchrotron XRD Measurements

Further development had been accomplished for verifying the coupons withthe Inconel 738 substrate 316 and thermal barrier coatings as theNiCoCrAlY bond coat 304 (FIG. 23 ), and a YSZ top coat and a layer ofYSZ and YSZ:Eu mixture in three configurations using the air plasmaspray (APS) process. Two multilayered thermal barrier coatingconfigurations were produced with the first having the doped layer atthe top of the undoped YSZ, and the second at the top of the bond coat304. Both configurations were fabricated along with the regular thermalbarrier coating configuration as a reference.

Synchrotron XRD measurements were taken at room temperature and under asingle flight thermal load up to 1100° C. The spatial distribution ofresidual strain in the YSZ layer of top coats of all the TBCconfigurations was quantified from the XRD data. The effect of theintroduction of the doped layer in the top coat of the TBC wasevaluated. Introducing the doped layer at the top of the bond coat didnot strongly affect the strain distribution, but the doped layer at thetop surface slightly altered the strain distribution in the YSZ layer.The strains at the surface of the TBCs were released upon heating. Inall the configurations, tensile in-plane residual strain was measureddue to the tensile quenching effect. The residual strain distributionsin the as-deposited TBCs indicated that the introduction of the dopedlayer did not affect the overall mechanical integrity of the coating.

Quantifying Thermal Barrier Coating Delamination Using LuminescenceModeling

Tracking damage and monitoring the integrity of thermal barrier coatingsover their lifetime prevents engine failure for gas turbines, reducesmaintenance costs and increases turbine system efficiency. It ispossible to apply infrared thermography to reveal delaminationprogression on a thermal barrier coating using the modified Kubelka-Munkmodel. It has been determined that luminescence-based imaging is anefficient delamination detection technique when doping the ceramic topcoat with luminescent rare-earth ions using the modified Kubelka-Munkmodel. This process allows for high-contrast and high-resolutiondelamination mapping to better monitor the integrity of enginecomponents that are exposed to extreme environments. Delaminationmonitoring is achieved through the tracking of luminescence intensity,to highlight areas of enhanced reflectance, corresponding to damagedlocations. It is possible to quantify luminescence contrast and detectearly stages of delamination and crack propagation using a shorterwavelength than those used in other infrared techniques. The modelevaluates delamination progression on luminescent multilayer thermalbarrier coatings.

Significant and unpredictable delamination on thermal barrier coatingsmay be caused by foreign object damage. For that reason, it is desirableto accurately predict the advancement of delamination on multilayerthermal barrier coating configurations. In accordance with thenon-limiting example, a 2×2-flux modified Kubelka-Munk model may be usedto estimate the luminescence intensity variations that are caused bylocal delaminations on these multilayer sensing coatings. The change indiffuse internal reflectivity due to the formation of an air gap at theinterface between the top coat and the bond coat may be used tocharacterize the delamination areas. To validate the results of themodel experimentally, an artificial delamination was created by Rockwellindentation on two distinct sensing layer configurations, experimentsconducted, and results validated.

Coating Manufacturing. The model outcomes were supported by concurrentlyfabricating two luminescent thermal barrier coating sampleconfigurations that embedded a YSZ:Er³⁺ layer as shown in FIGS. 32A and32B. Both configurations show a substrate 500 formed from one inch ReneN5 material as a nickel-based superalloy. A NiPtAl bond layer 502 isapplied over the substrate 500 and an undoped YSZ layer 504 as a topcoat is applied on the bond layer (FIG. 32A). Both configurations ofFIGS. 32A and 32B include a doped YSZ:Er layer 506. The doped layer 506is at the top surface (FIG. 32A) of the top coat 504, but the dopedlayer 506 may be at the bottom of the top coat 504 (FIG. 32B). Thesamples were formed from the Rene N5 superalloy substrate 500, theNiPtAl bond coat 502, a standard 125 μm, 8 wt % yttria-stabilizedzirconia (YSZ) Electron-Beam Physical Vapor Deposition (EB-PVD) thermalbarrier coating 504, and the additional 12.5 μm undoped EB-PVD layer forsubsurface delamination sensing having 0.8 mol % erbium-doped YSZ 506.The total amount of rare-earth stabilizer was kept constant at 8 wt % toensure the prominence of the metastable tetragonal phase of zirconia.For the first sample shown in FIG. 32A, the doped layer 506 wasdeposited over the undoped top coat layer 504 as illustrated.

Placing this sensing layer 506 as the doped layer as the top of thethermal barrier coating 504 increased the intensity of the luminescencethat can be collected by a detector, which facilitated data acquisition.This configuration in FIG. 32A also avoided direct interaction betweenthe bond coat 502 and the sensing layer 506, therefore avoiding largeuncertainties on interface properties in comparison with industrystandard bonding properties. The sensing layer as the doped top layer506 is placed at the top, and thus, becomes the upper surface of thethermal barrier coating and may also be used to indicate erosion becausethat sensing layer is directly exposed to extreme engine environments.

For the second sample shown in FIG. 32B, the doped layer 506 was placedat the lower side of the undoped coating 504 and interfaced with thebond coat 502. This coating configuration provided a better luminescenceintensity contrast for delamination assessment because the signaloriginated from the region that was impacted the most by the change inreflectance below the undoped coating 504. Because the as-depositedEB-PVD coatings were oxygen deficient, both samples were annealed at1000° C. for 3 hours in air. This helped hold the crystallinity andremove compounds containing the hydroxy group that could quenchluminescence.

In this example, the doped layer 506 is about 12.5 micrometers (X), butcan vary from 10.0 to 15.0 micrometers, and in another example, vary by5%, 10%, 15% or 20% from the 12.5 micrometer dimension and any values inbetween. The undoped layer 504 is shown as 125 micrometers thick (Y),but can vary from 110 micrometers to as high as 150 micrometers, and inanother example, may vary by 5%, 10%, 15% or 20% from the 125 micrometerdimension and any value in between those values.

An artificial delamination 509 was created by Rockwell indentation 510on both first and second samples. For the first sample (FIG. 32A), a2.45 kN load was applied normal to the surface and resulted in anobservable spallation of the coating at the indentation spot, anddelamination of the surrounding area of about 12 mm². This sphericalindentation load resulted in a central impression spreading to abutterfly, wing-like area resulting from anisotropic inelasticdeformation of the substrate (FIG. 35A). For the second sample (FIG.32B), a 1.96 kN load was applied and resulted in a lower lateral extentof delamination of the surrounding area of about 7 mm². Bothdelamination areas as generated by Rockwell indentation were intended toreplicate foreign object damage and produced a visible contrast to thenaked eye, suggesting a coating that buckled with a micron-sizeddelamination width.

Photoluminescence Measurements. Spectral acquisition was accomplishedusing a photoluminescence piezospectroscopy system 520 as shown in theschematic diagram of FIG. 33 for the system. The system 520 included afiber collection spectrometer 524 (Pixis 100, Princeton Instruments), asa collection probe or detector, operating under a 15 mW, 532 nm laserexcitation from laser 528. The collection probe 524 had a focal lengthof 7.5 mm, a depth of field of 2.2 mm, a numerical aperture of 0.27, anda spot size of 200 μm. The probe 524 was capable of fast scanning overthe surface of the sample 532 using an XYZ stage 536 with the collectionprobe 524 and the laser support 540 mounted on the XYZ stage. Acontroller 541 is operative with the XYZ stage 536, laser 528, anddetector as the spectrometer 524 for coordinating their operationalfunctions and collecting and processing data. This system 520 was usedalso for damage identification via piezospectroscopic stress evaluation,using the R-line emission of α-Al₂O₃ to obtain spectral characteristics,intensity ratio, stress quantification, and other data by comparingrare-earth emission spectra over the probed area.

This system 520 characterized thermal barrier coating stress usingluminescence from the thermally grown oxide (TGO). It is possible toincorporate a rare-earth ion such as from the bond coat as explainedbefore. The wavelength range was adjusted to 540-580 nm to collect theEr-lines and the system 520 was calibrated using a mercury lamp. Thethermal barrier coating samples as configured in FIGS. 32A and 32B weremounted on a vertical support 544 and aligned normally to the laser beam548. The spectral intensity was recorded over the surface of the samplesat spatial step increments of 200 μm. The emission peak at 562 nm,corresponding to the transition ⁴S_(3/2)→⁴I_(15/2) of erbium, was fittedusing a pseudo-Voigt model and a linear baseline removal. For largerarea measurements, it was possible to image directly and use a bandpassfilter for faster data collection time.

Model Formulations. The modified Kubelka-Munk model as explained abovewas used as the radiative transport model for high scattering media andnumerical estimation of light intensity distribution. This modelevaluated laser excitation and luminescence emission intensities andquantified delamination-induced luminescence contrast in thermal barriercoatings. For a given incident laser excitation intensity on the topsurface of the thermal barrier coating, the luminescence intensityemerging out at the top surface and emitted from the sensing layer wascalculated based on the diffuse internal reflectivity in the coating andthe absorption and the scattering properties, taken at the specificexcitation and emission wavelengths.

In accordance with a non-limiting example, two model cases wereevaluated as shown in FIGS. 34A and 34B. The first model (FIG. 34A)considered the thermal barrier top coat 503 containing two layers, i.e.,the sensing layer as the doped layer 506 and the regular undoped layer504 as the undoped YSZ and having the configuration similar to thatshown in FIG. 32A. The bond coat 502 was adjacent the undoped layer 504,but could be an air gap in this example. The sensing layer 506, whichcontained low levels of dopant, was assumed to include the same opticalproperties as standard undoped EB-PVD YSZ, such as corresponding to theundoped layer 504. The ceramic top coat 503 formed by the YSZ wasassumed to be isotropic. This first model provided an initialapproximation, but some low dopant concentration and agglomeration mayhave resulted in a substantial modification of absorption and scatteringproperties. These numerical estimations using the isotropic assumptionwere expected to accord with real conditions and in relativelyhomogeneous microstructures, such as plasma spray coatings.

For EB-PVD coatings, the assumption on uniform scattering and absorptioncoefficients through the thermal barrier coating thickness was addresseddue to the waveguide-like scattering produced by the widening columnarmicrostructure that formed from small equiaxed grains at the base of thetop coat 503, where scattering was expected to be much stronger thannear the coating surface. Therefore, the second model as shown in FIG.34B focused on creating more accurate quantitative predictions, andincorporating three layers in the top coat 503 that included a highscattering area 510 adjacent the bond coat 502 to account for thewell-defined two-zone anisotropic microstructure of EB-PVD coatings andfor the luminescent layer.

A representation of the Kubelka-Munk model shown in FIG. 34A includedboundary conditions and definitions and included the doped layer 506 asthe top surface of the thermal barrier coating, and more particularly,over the undoped EB-PVD YSZ layer 504. In this model, light traveledalong the x-axis, corresponding to the normal to the surface of thethermal barrier coating, in two opposite directions. The light vectorcomponent directed towards the bond coat 502 was denoted by I and thelight vector component directed back to the top surface of the thermalbarrier coating was denoted by J as shown in both FIGS. 34A and 34B. The2×2-flux model differentiated both excitation and emission wavelengthsto solve for the laser and luminescence intensities separately.Similarly, generalized absorption and scattering coefficients had beenestablished, k and s, respectively, and defined for a specificwavelength (labeled A).

It should be understood that many of the high-level mathematics used inthe model are summarized below in general terms since the details arenot necessary for understanding and also representative equations andmatrices set forth in FIGS. 34C to 34J. Those skilled in the art willreadily appreciate that high level mathematics are not necessary forunderstanding the modeling applied to the processing for determiningdelamination in these examples. The column vector Y_(λ)(x)=[I_(t,λ)(x)J_(t,λ)(x) I_(b,λ)(x) J_(b,λ)(x)]^(T) defined radiation intensities anda matrix was established to determine optical properties in a specificlayer z (top layer z=t or bottom layer z=b) (FIG. 34C). The intensity oflaser light as a function of the depth in the coating was thencalculated. The luminescence was assumed to occur exclusively due to theexcitation of the sensing layer as the doped layer by the laser, whichwas given by I_(l) or J_(l). A matrix was formulated that defined theamount of luminescence generated in the specific layer z (FIG. 34D).

The matrix included components q_(z) as the quantum efficiency of thelayer z. If the layer was luminescent, q_(z)=0.5, and if not, thenq_(z)=0. As an example, for the first sample (FIG. 32A and referred toin the model of FIG. 34A as a two-layer model), q_(t)=0.5 and q_(b)=0.It was then possible to solve for the distribution of the luminescenceintensities in the coatings.

The second model study (FIG. 34B) included a high scattering area 510 toform a three-layer model, which expanded the complexity of the model byconsidering three layers in the thermal barrier coating top coat 503, asrepresented by the model shown in FIG. 34B. At the lower side of theundoped coat 504 and adjacent the bond coat 502 (or an air gap), theshaded zone corresponded to the high scattering area 510, whichrepresented the EB-PVD microstructure. A column vectorY_(λ)(x)=[I_(t,λ)(x) J_(t,λ)(x) I_(m,λ)(x) J_(m,λ)(x) I_(b,λ)(x)J_(b,λ)(x)]^(T) defined the radiation intensities. The intensity of boththe laser and luminescence light as a function of coating depth wassolved, respectively (FIG. 34E).

There were certain model parameters. Generalized coefficients k and sthat accounted for the bidirectional path of light were calculated fromthe absorption and scattering coefficients, respectively k and s, givenin Table 2, and such that K=2k and S=2s, corresponding to isotropicbackscattering with no forward scattering. The high scattering area wascharacterized by a stronger diffusion of light, which can be simulatedusing s′>s and where k remained unchanged inside the entire thermalbarrier coating top coat 503 defined by the doped layer 506, undopedlayer 504, and high scattering area 510. The thickness of the highscattering zone was fixed to 10 μm, based on reported values ofmicrostructure.

TABLE 2 Scattering and Absorption of As-Deposited EB-PVD YSZ Scatteringcoefficient s Absorption coefficient λ (nm) [s′] (m⁻¹) k (m⁻¹) 53212,965 [46,015] 407 562 12,107 [39,336] 319

The boundary conditions are defined such that, at the top surface, thepercent intensity of the incident laser light was set toI_(l)(x=0)=I₀=100%, and there was no external luminescence input at thesurface, so that I (x=0)=0%. Depending on the interface type consideredat the bottom or lower section of the top coat 503, either with an airgap, in the case of a delamination, or with the bond coat 502, for anintact coating, the reflectivity ρ_(i) was set to a specific value. Forthe case corresponding to delamination 509, the diffuse externalreflectivity at the interface between the top coat 503 and an air gapwas defined. An expression was obtained by applying an integratedaverage of Fresnel equations to obtain a diffuse internal reflectivityρ_(i), at the interface between the top coat 503 and the air gap, whichwas used as a boundary condition for the model (FIG. 34F). It should benoted that the diffuse external reflectivity equation is similar to thatof FIG. 13 .

In the model calculations, ρ₀ was considered the diffuse externalradiation at the interface between the top coat 503 and the air gap, andρ_(i, max) was the maximum diffuse internal reflectivity andn=n_(YSZ)/n_(air) was the ratio of refractive indices, which compliedwith the condition n≥1. The refractive index of the EB-PVD YSZ is givenin Table 3 below and by definition, n_(air)=1. The value of reflectivityobtained at 532 nm is 82%, and at 562 nm, it is 83%, for the case of adelamination 509 for which the air gap width was assumed to be largerthan the signal radiation wavelength, which enabled maximum reflectivityand discarded frustrated reflectance. The formation of the thermallygrown oxide at this location after aging of the thermal barrier coatingmay contribute substantially to a change of reflectivity, estimated tobe approximately 39% at the interface top coat/TGO, with α-Al₂O₃ wheren_(TGO)=1.76.

TABLE 3 Refractive Index of EB-PVD YSZ λ (nm) n 532 2.17 562 2.16

The reflectivity for the interface between the top coat 503 and the bondcoat 502 was estimated using the model for the frustrated angle-averagedeffectively for radiation with an angle of incidence greater than thecritical angle θ_(c) of about 27 degrees (FIG. 34G). A variable d in thecalculations was the air gap width formed through delamination and θ wasthe angle of incidence, φ was an azimuthal angle and α was given for aperpendicular polarization and for a parallel polarization β (FIG. 34H).

In further calculations, the λ₀ was the radiation wavelength andn=n_(YSZ)/n_(air). The frustrated angle-averaged reflectivity forunpolarized radiation was found (FIG. 34I), and in the calculations,components were used for the frustrated angle-averaged reflectivitiesfor perpendicular and parallel polarized radiation, respectively, andwere calculated using α=α_(⊥) and α=α_(∥). The air gap width dependentdiffuse internal reflectivity, which can be of particular interest, forexample, for the examination of early stages of delamination, wasobtained (FIG. 34J).

The numerical value of the reflectivity at the interface between the topcoat 503 and the bond coat 502 was obtained, and taking the limit as dgoes to 0, the reflectivity was found to be about 4% at 532 and 562 nmwavelengths.

Experimental Measurements. Images of the coatings with Rockwellindentation-induced delamination or spallation areas for the doped layer506 at the top surface (FIG. 32A) and doped layer at the bottom of thetop coat 503 (FIG. 32B) are shown in FIGS. 35A and 35B. The indentation510 and delamination 509 area for both are illustrated, and with alarger spallation area in FIG. 35A for the doped layer 506 at the topsurface 503. In both samples, delamination areas were detected due tothe larger and sharper increases in luminescence intensity than in theincrease in reflectance observed by the naked eye. The thermal barriercoatings also showed some defects that could be detected with thisprocess because the defects were associated with a luminescenceintensity that was significantly lower than collected in an intact zoneof the coating.

For the first sample (FIG. 32A) having the doped layer 506 at the topsurface, distinct locations close to the indent area have top surfacemarkings corresponding to contamination as shown in the image of FIG.35A, causing a local reduction in the emission intensity. Spallation 530also resulted in zero luminescence in those areas. Other factors thataffected measurements include: (a) irregularities in the coatingthickness, (b) the concentration variation and dispersion in-homogeneityof the luminescent ions in the material concentration, (c) curvature ofthe thermal barrier coating along the turbine blades, and (d) partialerosion, sand glazing or calcia-magnesia-alumina-silicate surfacedeposition. These factors contributed to luminescence intensityinconsistencies or surface coating opacity in localized areas.

In the first sample of FIG. 32A with the doped layer 506 as the topsurface, although this doped layer as the sensing layer was not indirect contact with the bond coat 502, the increase of diffuse internalreflectivity at the bottom of the ceramic top coat 503 was associatedwith a debonding at the interface between the top coat and the bond coat502 and produced a noticeable increase of luminescence intensity. Forthe second sample of FIG. 32B with the doped layer 506 as the bottom ofthe top coat 503, even though the indentation load was modestly lowerthan that of the first sample, smaller lateral delamination expansionwas generated as shown in the image of FIG. 35B. A considerable contrastwas observed, indicating the particular effectiveness ofluminescence-based measurements for this layer configuration. Theseluminescence intensity contrasts were due to the increased reflection ofthe laser and luminescence lights at the interface between the top coat503 and the bond coat 502 in presence of an air gap, providing moreexcitation to the sensing layer and more reflected luminescence toemerge out of the thermal barrier coating surface.

In both samples, the indentation location showed a reduced luminescenceintensity resulting from the possible compaction of the doped coat 506as the sensing layer. Moreover, for both first and second samples, therewas enough luminescence intensity contrast for fast delaminationdetection, which is advantageous with multilayered configurations, suchas shown in FIGS. 32A and 32B. For the application of this delaminationmeasuring technique for delamination detection on curved surfaces, suchas some turbine blades, luminescence intensity gradients may be expectedand configurations similar to the second sample having the doped layer506 at the bottom of the top coat 503 (FIG. 32B) with high luminescenceintensity contrast may be preferred. It should be understood that thedopants, concentrations, ranges, thickness dimensions and other factorsmay be similar as to other examples described above relative to FIGS.1-35A and 35B.

Two-Layer Model. As noted before, a two-layer model was implemented andstudied. The solution obtained by the modified Kubelka-Munk model forthis first case study where two layers were considered, i.e., thesensing layer 506 and the other undoped layer 504 as shown in FIG. 34A,provided the distribution of light intensities. At any point in thecoating, the respective intensities I and J for laser (l) andluminescence (L) were calculated based on absorption, scattering and theinterface reflectivity using the modified Kubelka-Munk model. Theresults for the laser intensity are shown in the graph of FIG. 36 ,showing the distribution of laser intensity in the two-layer model (FIG.34A). In the case where there is delamination, the higher intensitiesare scattered back due to increased reflectivity.

The location of the doped layer for the first and second samples (FIGS.32A and 32B) are shown and labeled corresponding to the doped layer 506at the top surface on the left side of the graph, and the doped layer atthe bottom of the top coat 503 as noted on the right side of the graph.In the presence of delamination 509, there is a substantial differencein the intensity of light traveling back to the top surface (J_(l)). Theadditional excitation intensity in the doped layer 506, particularlyenhanced in regions closer to the interface between the top coat 503 andthe bond coat 502, contributed to the higher intensity of luminescenceproduced in the sensing layer 506. For the first sample having the dopedlayer 506 at the top surface (FIG. 32A), the increase of the intensityof the backscattered laser radiation J_(l) contributed substantially tothe gain in excitation intensity that was available for the productionof luminescence, in the case of delamination.

For the second sample having the doped layer 506 at the bottom of thetop coat 503 (FIG. 32B), the summation of the integrated intensities ofthe incoming laser radiation I_(l) and the backscattered laser radiationJ_(l) was increased in the presence of a delamination, largelycontributing to the higher intensity of luminescence. The results of themodified model for the luminescence intensities for the first and secondsamples corresponding to those illustrated in FIGS. 32A and 32B, i.e.,either the doped layer 506 at the top surface or doped layer at thebottom of the top coat 503, are shown in the graphs of FIGS. 37 and 38 ,respectively, with doped layer to top shown at the left side of thegraph in FIG. 37 , and doped layer at the bottom of the top coat shownat the right side of the graph in FIG. 38 . The dotted areas in eachgraph either on the left represent the location of these doped layers inthose respective samples. There was an overall increase of luminescenceintensity when a delamination was present due to both the higher amountof laser excitation energy available and the greater reflection of theluminescence radiation itself, at the bottom of the top coat 503.

Three-Layer Model. As noted before, a three-layer model was implementedas shown in FIG. 34B, which was an improvement over the two-layer modelbecause it integrated a high scattering layer 510 located at the base ofthe top coat 503. This high scattering area 510 had distinct opticalproperties as observed on typical EB-PVD microstructures. Thisthree-layer model (FIG. 34B) produced more accurate results incomparison with the two-layer model (FIG. 34A) because the three-layermodel captured the differences in optical properties associated with theevolving microstructure of EB-PVD coatings with depth. The three-layermodel provided a distribution of laser intensity as shown in the graphof FIG. 39 , illustrating also the locations of respective samples as inFIGS. 32A and 32B and the high scattering area 510. The results for theluminescence intensities for the first and second samples are shown inthe graph of FIGS. 40 and 41 , respectively. In the case where there wasdelamination, the higher intensities were scattered back due toincreased reflectivity.

The sensitivity to layer thickness variations that exists in practicewas taken into account to calculate the standard deviation error resultsfor the calculation of J (x=0), which represented the measurableluminescence intensity that emerged from the top surface and wascollected at the detector. The numerical values found for J (x=0) arereported in Table 4. Generally, the EB-PVD fabrication technique leadsto better control on the thickness of the deposited coatings, with aprecision that can be as low as 1 μm. Assuming that the overall top coatthickness remained constant, e.g., about 137.5 μm, the model was solvedfor a sensing layer thickness of 12.5±1 μm.

TABLE 4 Results of the Kubelka-Munk Model for the Predicted LuminescenceIntensity Emerging out at the Surface Intensity (a.u.) Sample A Sample BTwo-layer model J (x = 0) delamination 0.764 ± 0.057 0.198 ± 0.017 J (x= 0) no delamination 0.683 ± 0.049 0.017 ± 0.003 Three-layer model J (x= 0) delamination 0.768 ± 0.057 0.196 ± 0.019 J (x = 0) no delamination0.703 ± 0.052 0.031 ± 0.005

TABLE 5 Modeling and Experimental Results for the Ratio of DelaminationOver Intact Coating Luminescence Intensities Enhancement factor η SampleA Sample B Two-layer model 1.12 ± 0.16 11.36 ± 2.98  Three-layer model1.09 ± 0.16 6.32 ± 1.63 Experimental measurement 1.17 ± 0.02 4.82 ± 0.47

Comparison of Results. Table 5 above compares the modeling andexperimental results. The luminescence intensity contrast (orenhancement factor η) between a delamination area and an intact coatingarea was obtained in the models by juxtaposition of the two extremecases of diffuse internal reflectivity, e.g., 4% for an intact coatingand 82-83% in the presence of a delamination area. The error shown inthe modified Kubelka-Munk models accounted for typical thicknessvariation. Experimentally, the enhancement factor η was obtained bydividing the average of 10 measurements in the delamination area 509 bythe average of 10 measurements outside this area. The reported errorcorresponded to the standard deviation over these points.

This comparison indicated that, as expected, the three-layer model (FIG.34B) better correlated with experimental measurements. For the firstsample having the doped layer 506 at the top surface (FIG. 32A), theenhancement factor η obtained from both modeling approaches was close tothe experimental result. However, for the second sample having the dopedlayer 506 as the bottom of the top coat 503 (FIG. 32B), the two-layermodel predicted an enhancement factor well above the actual experimentalresults, which indicated that the sensing layer 506 of this secondsample presented a different and stronger scattering behavior whencompared to the rest of the thermal barrier coating.

Because much of the contrast is produced by multiple scattering events,i.e., both laser and luminescence in the doped layer as the sensinglayer 506, the larger mismatch with the prediction is expected for thesecond sample where the doped layer is at the bottom of the top coat 503(FIG. 32B). The three-layer model as shown in FIG. 34B accounts for thishigh scattering at the base of the thermal barrier coating, and in thiscase, corresponds to the base of the doped layer as the sensing layer506, and generated a more accurate estimation and validated thismodeling approach for EB-PVD coatings. It was found that the three-layermodel was appropriate for estimations on the thermal barrier coatinglayer configuration in the second sample where the doped layer 506 is atthe bottom of the top coat 503 (FIG. 32B).

The modified Kubelka-Munk model helps characterize the progression ofcoating delamination. The coating health may be monitored by measuringluminescence intensity over extended areas, which is helpful forimportant commercial applications, including gas turbine blades. Thelocations where luminescence contrast exceeded the lower boundary of theenhancement factor predicted by the models indicated the delaminationzones. Some of the assumptions and simplifications on the Kubelka-Munkmodels and the accuracy of the coefficients selected for the materialmay contribute to discrepancies between model estimations and whatactually occurs. However, the system and method as described hasobtained substantially accurate results in a reliable manner.

Delamination Model Capabilities and Extensions. The evaluation of theluminescence enhancement factor and the signal intensity trade-offsshowed that having the sensor layer or doped layer 506 on the bottom ofthe thermal barrier coating (FIG. 32B) was preferable for delaminationmonitoring. This process may also be used for optimizing the thicknessof the sensing layer 506 at the bottom of the top surface 503. The graphof FIG. 42 shows there are some signal trade-offs for an example 137.5μm EB-PVD thermal barrier coating configuration containing a luminescentlayer with varying topology. In the graph, the position axis indicatesthe location of the top of the sensing layer as the doped layer 506embedded in the thermal barrier coating, while accounting for the highscattering zone at the base of the coating (10 μm), as given in thethree-layer model.

The enhancement factor η corresponded to the achievable luminescencecontrast generated in the presence of a delamination, and confirmed thatvery thin sensing layers as the doped layers 506 located at the bottomof the top coat 503 (FIG. 32B) were more ideal for maximum contrast indetection. Luminescence intensities were normalized to allow for directintensity comparisons between distinct layer configurations, whileavoiding inaccurate numerical estimations that may originate from theinherent uncertainty on quantum efficiency. A thicker sensing layer asthe doped layer 506 provided more intensity, but with some contrastloss, e.g., less contrast from portions of the sensing layer that weredistant from the interface. Additionally, although sample configurationswith a sensing layer as the doped layer 506 at the top surface (FIG.32A) provided high luminescence intensities, they were vulnerable toerosion since the sensing layer was exposed. However, as a benefit, forthat reason, the system and process as described may be used to monitorerosion using the luminescence-based modeling methods as presentedabove.

A configuration that may be operable for multi-purpose detectioncapabilities may be a fully doped coating, with a higher luminescenceintensity and higher contrast than the other configurations and with thesensing layer 506 placed on top of the regular EB-PVD YSZ coating (FIG.32A). The effects of frustrated total internal reflectance that occurredfor air gaps smaller than the radiation wavelength may be studied. Thesubstantial variations of diffuse internal reflectivity may be used tocharacterize the early stages of delamination during formation of airgaps. Similarly, thermally grown oxide growth and location ofdelamination with respect to this oxide layer may be evaluated using themodels explained above.

It is possible to evaluate the trade-offs between luminescence signalstrength and the delamination contrast that can be used to optimizedoped layer coating configurations, e.g., two-layer with the sensinglayer or doped layer 506 and regular undoped layer 504, and three-layerwith a high scattering area 510 integrated to the previous case. Themodels helped predict, for any thermal barrier coating configurationthat embeds a luminescent layer, the luminescence intensity contrastthat may be used to quantify and monitor delamination areas in thesethermal barrier coatings.

Modeling results were compared to experimental values that werecollected on two as-deposited EB-PVD thermal barrier coatings, whicheach contained an erbium-doped YSZ layer for delamination sensing. Anartificial delamination zone created by Rockwell indentation wassuccessfully quantified by measuring the intensity of the erbiumemission at 562 nm over the surface of the coatings. The luminescencecontrast predicted by the three-layer model was found to be in goodaccordance with experiments, emphasizing the importance of consideringthe microstructure anisotropy in EB-PVD thermal barrier coatings foraccurate delamination characterization. This modeling approach may aidto help determine signal trade-offs for layer topology optimization. Thesystem and process may be applied to different coating depositionmethods to evaluate the early stages of delamination progression inthermal barrier coatings, and to assist coating health monitoringmeasurements and to facilitate safer thermal barrier coating operationand improved lifetime.

Revealing Temperature Gradient Across Thermal Barrier Coatings

The thermal barrier coatings as described may be used in combinationwith air cooling systems to protect metal substrates from extremetemperatures in high-pressure turbines as an example. Temperature rangesin high-pressure turbines may vary from 1300° C. to 1600° C. Air filmcooling may provide a change in temperature from −100° C. to −400° C. Atthe thermal barrier coating, the change in temperature may range from−150° C. to −200° C. In major applications, such as jet turbine andpower generation engines, this change may impact engine performance andmaintenance schedules. For example, many components besides turbineblades in an aircraft engine include thermal barrier coatings. Typicalcomponents, for example, of a General Electric turbofan engine, such asthe GE9X, include special alloys and ceramic matrix composites. Theengine has a higher bypass ratio and compression ratio than many otherengines to improve the fuel ratio. This type of engine includesimprovements in the fan unit, low-pressure compressor, high-pressurecompressor, combustor, low-pressure turbine, high-pressure turbine, andturbine blades. Many of these components are coated with a thermalbarrier coating to protect them in extreme operating environments.

As noted before, state-of-the-art thermal barrier coatings have not beenused to their highest potential because of uncertainties in thetemperature measurements at high-temperature operation of the variousthermal barrier coated components, such as turbine blades. Safetymargins as high as 200° C. exist, and the ideal Brayton cycle efficiencyis dependent upon the temperature ratio of the compressor exit andturbine inlet where these safety margins can be important. A 1%efficiency improvement can save millions of dollars in fuel over acombined-cycle plant life, and a 130° C. increase may lead to about a 4%increase in engine efficiency. Failure mechanisms are often driven bytemperature conditions in the depth of the thermal barrier coating.

A more accurate determination of thermal gradients in thermal barriercoatings would allow a more safe and more efficient operation of variouscoated components, such as in gas turbine engines. Failure mechanisms ofcoated components may be thermally activated during engine operation,and the uncertainty in temperature measurements may contributesignificantly to their lifetime uncertainty.

Thermal barrier coatings on many components, such as in gas turbineengines, operate in the presence of large thermal gradients that existthroughout their thickness. For example, a large temperature gradientmay exist through the thickness of a thermal barrier coated turbineblade during high temperature, peak operation. The weaker portion of thethermal barrier coating may be located near the top coat and bond codeinterface, creating a weaker area that may affect component performanceand longevity. The system as developed determines a more precisesub-surface location of phosphor thermometry measurement points and hasbeen implemented to determine thermal gradient and temperatures at theinterface and throughout the thickness of the component from the topcoat of the thermal barrier coating through the bond coat and to thesubstrate.

Referring now to FIG. 43 , an example component 600 having a thermalbarrier coating 602 is illustrated on a substrate 606 formed from alloy247, also referred to as AMR-N 247, which may be provided as a castpolycrystalline nickel-base superalloy developed by Martin MariettaCorporation, and often investment cast, but also useful with directionalsolidification techniques to improve the creep rupture strength. TheNiCrAlY bond coat layer 610 may be formed over the substrate 606 and aceramic top layer 614 as the YSZ:Er top coat layer applied on the bondcoat layer and formed by techniques such as air plasma spraying. Theceramic top coat layer 614 in this example may include 1.5% erbium. Inthis example, it was annealed for 2 hours at 800° C. The substrate 606is about 3 millimeters thick for experiment purposes, but can vary inthickness depending on end-use applications, such as when the substrateis a turbine blade. The bond coat 610 in this example is about 120 toabout 150 micrometers thick, and the ceramic top layer 614 is about 160to about 180 micrometers thick, and both may be applied by air plasmaspraying or other techniques. The apparatus for measuring thermalgradients across the thermal barrier coating 602 is illustratedgenerally at 630 and includes a laser 634, detector 638, and acontroller 640 operatively connected to the laser and to the detectorfor controlling their operation. The laser 634, detector 638, andcontroller 640 may be similar to other components described above in thedescription with FIGS. 1-42 .

In accordance with a non-limiting example of the system and method, thethermal gradients shown by the varying T in FIG. 32 may be characterizedthrough various translucent materials, including thermal barriercoatings for different end-use applications, such as gas turbineengines. The luminescence decay apparatus and methodology describedabove and associated with phosphor thermometry may improve gradienttemperature monitoring through a thermal barrier coating and increasethe operational lifetime of different substrate materials, such as aturbine blade, and increase overall efficiency of engines. The phosphorthermometry decay signal is analyzed to reveal thermal gradients in thedepth of the thermal barrier coating or other translucent materials.Although the description will proceed relative to thermal barriercoatings, it is possible the system and described methodology may beused on many different translucent materials. The data collected at thedetector 638 is analyzed and processed at the controller 640 and thethermal profile across the depth of the thermal barrier coating 602 isdetermined. As shown in FIG. 43 , there is a varying temperature (T)across the thermal barrier coating 602 and to the substrate 606, and inthe presence of a thermal gradient across the thermal barrier coating,the collected convoluted luminescence decay signal may be used toreconstitute and determine through-the-depth temperature points.

This data may be used for improved precision of temperature measurementsin extreme temperature environments. The resulting decays may becomputed at the controller 640 via a software module that convertsdecays to thermal gradients. The capability to obtain data to monitorthe thermal gradients instead of only the surface temperature or pointmeasurements provides a better insight for the operation of differentcomponents that have applied protective thermal barrier coatings, suchas 602 in FIG. 43 , to improve design options, which are supported bythe more precise data regarding the measurement of temperature gradientsacross the thermal barrier coatings. Better control of remotetemperature measurements in extreme operating environments may beapplied to other materials, such as cryogenic materials, combustioncomponents, and turbine engine components. Thus, not only may a singlevalue for a temperature measurement be obtained for each probedlocation, but a temperature distribution extending through the material,such as the thermal barrier coating 602 applied on a substrate 606,throughout the depth of the probed point may be obtained. Athree-dimensional (3D) temperature map may be obtained, such as byobtaining the two-dimensional surface temperature map, and anotherdimension through the depth of the material at each surface point.

Referring now to FIG. 44 , a graph of the emission spectrum for the topcoat 614, as an example YSZ:Er top coat as doped with erbium and at 532nanometer (nm) excitation is illustrated, showing the 560 nm bandpasswith the dashed lines indicated by “A” and YSZ:Er spectrum and thedouble peak indicated by solid lines at “B.”

Referring now to FIG. 45 , there is illustrated a graph withcorresponding equations illustrating how the gradient of temperature maybe calculated using the thermal conductivities, where the summed R forthe thermal top coat 614, thermal bond coat 610, and thermal substrata606 are added for the total gradient of temperature. The temperaturegradient from the top surface of the top coat 614 down the bond coat 610to the substrate 606 is shown by the line indicated generally at 650.

The YSZ top coat 614 in this example includes the rare-earth dopedmaterial, such as erbium, that exhibits multi-exponential decays andincludes distinct crystallographic sites resulting in multiple decaytime constants. To measure the thermal gradient in the thermal barriercoating 602, cross-relaxation and laser pulse power may impart greaterimportance and the impurities and dopant agglomeration may add to adelayed starting, fitting window to reduce the effect of fast-decayingcomponents. In system operation, a constant time window may be used formeasurements that are independent of the settings of a data acquisitionsystem, such as observation length and sample rate. The YSZ:Er materialwas chosen for its decay because it was close to an idealsingle-exponential decay with one dominant crystallographic site. In theabsence of a thermal gradient, the start and end times of the fittingwindow do not affect the lifetime decay measurement in this type ofthermal barrier coating configuration. When there is a thermal gradient,the start and end times of the fitting window may become moreapplicable.

In operating conditions, a temperature gradient exists in thermalbarrier coatings, such as used on turbine blades. The emergingluminescence as described above is a convoluted signal coming from allthe locations in the doped layer. Varying the fitting window size of theacquired signal will allow multiple temperature measurements to beacquired throughout the depth of the thermal barrier coating 602 to thesubstrate 606.

Referring now to the graph in FIG. 46 , there is illustrated thenormalized intensity versus the time, where a short fitting windowcontributes a greater amount to the intense, fast decay from dopantsthat are exposed to the higher temperature on top surface locations. Thelonger fitting window contributes a greater amount to the low intensity,long decay from further inside dopants that are exposed to lowertemperatures. This contrast is illustrated on the decreasing decay linerelative to increased time along the horizontal axis in the graph ofFIG. 46 .

It should be understood that the location that is measured usingphosphor thermometry is reported for different temperature gradients andsurface temperatures (T₀). It is possible to predict the sub-surfacelocation of the temperature point as the YSZ:Er decay has littlesensitivity to thermal parameters.

Referring now to FIG. 47 , there is illustrated another more detailedschematic of the apparatus 630 that may be used for the determination ofreference decays and showing a muffle furnace 652 and the sample 600within the furnace in an isothermal case. The apparatus 630 includes thecontroller 640 that operates with a laser 634, such as a pulsed 532nanometer (nm) laser source, and a detector 638 as a photomultipliertube in this example that includes a 560 nanometer bandpass filter. Thelaser 634 generates the laser pulse onto a 532 nanometer laser mirrorsystem having two laser mirrors 660 a, 660 b as illustrated and into adichroic filter 664, and in this example, a cyan dichroic filter 666with a 30° AOI and through a convex lens 668 into the sample 600, whichin this isothermal case is contained within the muffle furnace 652 toisolate the sample from fuel and all products of combustion, includinggases and flying ash. The muffle furnace 652 may be electricallypowered.

The sensor as detector 638 response may be calibrated in associationwith the furnace 652, which in an example test included through holesfor a thermocouple and for taking luminescence measurements. As to the“fit,” the temperature-dependent multi-phonon relaxation model for thetransition may be combined with a model to account for the otherthermally populated levels. It is possible that the muffle furnace 652is not used, and instead, employ a burner rig nozzle 670 that creates agas plume onto the sample 600 for a thermal gradient case experiment.

Referring now to the graph of FIG. 48 , there is illustrated a graph ofthe reference decays at 562 nanometers for the YSZ:Er coating. The lasersource 634 includes Nd:YAG 532 nm laser operating at a 0.5 mJ pulseenergy and at 10 Hz. The time is shown on the vertical axis andtemperature on the horizontal axis.

It is possible to measure decay in a thermal gradient case by includinga burner rig nozzle 670 as illustrated and having no muffle furnace 652.The burner rig nozzle 670 imparts a gas plume onto the sample 600. Againthe laser 634 may be a Nd:YAG laser using a neodymium-doped yttriumaluminum garnet as a crystal for the laser medium. In another example,the dopant may be a triply ionized neodymium that replaces a smallfraction, e.g., about 1% of the yttrium atoms in the host crystalstructure of the yttrium aluminum garnet (YAG) since the two ions are ofsimilar size. The system may include an infrared camera and athermocouple that are not illustrated in detail and operative forsurface and opposite side temperature measurements.

Referring now to the graph of FIG. 49 , there is illustrated thenormalized intensity versus time for the YSZ:Er at 560 nanometers andshowing time on the horizontal axis versus normalized intensity on thevertical axis and the graph lines labeled A-I and the correspondingtemperature.

Referring now to FIGS. 50 and 51 , there are illustrated graphs showingthe results for the phosphor thermometry sub-surface measurements. Asingle-shot laser pulse and associated measurement may be used in thisexample at a given surface temperature. The fitting window was variedand the thermal gradient was revealed across the thermal barrier coatingthickness. The sub-surface location measured by phosphor thermometry maybe verified using an infrared and thermocouple measurement.

As shown in FIG. 50 , the reference is shown with no temperaturegradient and labeled “A” and the measurements with the temperaturegradient are illustrated at “B.” The graph shows the variation. Thelifetime decay in microseconds is shown along the vertical axis and thesurface temperature along the horizontal axis. The sub-surface at 565°C. is illustrated and its point along the graph shown.

Referring now to FIG. 51 , there is illustrated the through-thicknessthermal profile with the measured temperature in Centrigrade and thesurface temperature and the window size in percent relative to thereference. The window size percentage corresponds approximately to thesub-surface measurements from 0 to 70 micrometers using the Kubelka-Munkmodeling.

A precise determination of temperature in the thermal barrier coatings602 may result in large benefits, such as with gas turbine engines, forfuel savings, reduced emissions, and better lifetime monitoring of thethermal barrier coating. A single-exponential decay fit may be used asapproximations for practical temperature measurements using phosphorthermometry on luminescence lifetime decay of the YSZ:Er. Variation inthe fitting window size results in successful attainment of sub-surfacetemperatures in the doped thermal barrier coating. The phosphorthermometry measurements in the presence of a thermal gradients showpromising results to reveal sub-surface temperatures and obtainthree-dimensional temperature profiles of the thermal barrier coatings.

Referring now to FIG. 52 , there is illustrated a composite drawing oftemperature profile at 670, schematic thermal barrier coating at 674,and shorter window and longer window graphs at 676, 678. The drawingshows how the acquisition window changes to retrace temperaturegradients into the thermal barrier coating. The model provides twotemperature measurement locations as illustrated with the thermalbarrier coating surface configuration at 674 showing the depth inmicrometers and the temperature differential at those two points. Thetemperature profile shows the shorter window time period of 50-60 microminutes and the longer window of 60-85 micro minutes, and the associatedgradient and temperature. Two temperature values are provided and theexperiment is accomplished with YSZ:Er and the unknown temperaturegradient. The graphs 676, 678 at the bottom of FIG. 52 show therespective shorter window in an example of 3.236 microseconds, and thelonger window in an example of 3.357 microseconds and the time period inmicroseconds. The error values shown are based on a 95% confidenceboundaries for the data fit. Additional error may be carried by thereference fit.

In a method aspect, the acquisition signal for one laser shot may bedecomposed for the different window periods and the software at thecontroller analyzes the temperature values. It is also possible to usemultiple laser shots with each laser shot having a different window foracquisition and measurements.

Many modifications and other embodiments of the invention will come tothe mind of one skilled in the art having the benefit of the teachingspresented in the foregoing descriptions and the associated drawings.Therefore, it is understood that the invention is not to be limited tothe specific embodiments disclosed, and that modifications andembodiments are intended to be included within the scope of the appendedclaims.

The invention claimed is:
 1. A method of forming a thermal barriercoated component, comprising: applying a metallic bond coat layer onto asubstrate, said metallic bond coat layer including about 96 to 98percent of NiCoCrAlY and about 2.0 to about 4.0 percent of rare-earthluminescent dopants selected from the group consisting of samarium,erbium, europium, dysprosium, praseodymium, and terbium; applying aceramic top coat layer on the bond coat layer; and forming a heat aged,high temperature sensing thermally grown oxide (TGO) layer at theinterface of the bond coat layer and ceramic top coat layer by heatageing the metallic bond coat layer and ceramic top coat layer tomigrate rare-earth luminescent ions selected from the group consistingof samarium, erbium, europium, dysprosium, praseodymium, and terbiumfrom the metallic bond coat layer into the interface of the bond coatlayer and the ceramic top coat layer in an amount sufficient to enablehigh temperature luminescence sensing up to about 1,100° C. of the TGOlayer for real-time phosphor thermometry temperature measurements at theTGO layer, wherein said TGO layer comprises about 1.7 to 2.0 weightpercent of nickel, about 0.67 to 0.82 weight percent of chromium, about52.7 to 64.4 weight percent of aluminum, about 35.0 to about 42.7 weightpercent of oxygen, and no more than about 0.1 weight percent of therare-earth dopants selected from the group consisting of samarium,erbium, europium, dysprosium, praseodymium, and terbium.
 2. The methodof claim 1, wherein said ceramic top coat layer comprises aytrria-stabilized zirconia (YSZ) barrier top coat layer on the bond coatlayer.
 3. The method of claim 1, wherein said substrate comprises aturbine component or an engine exhaust component.
 4. The method of claim1, wherein said bond coat layer is about 50 to 200 micrometers and saidceramic top coat layer is about 50 to 300 micrometers.
 5. The method ofclaim 1, wherein the substrate comprises a superalloy substrate.